i2c: core: Add i2c_update_timeout() helper for dynamic transfer timeouts

The transfer timeout for an I2C controller should reflect the actual
message length and bus frequency rather than a static 1-second value.
A static timeout causes unnecessary delays on error paths for short
messages, and may be insufficient for very long transfers.

Add i2c_update_timeout() to i2c-core which computes a transfer-specific
timeout and stores it directly in the standard adap->timeout field.  The
formula accounts for 9 bits per byte (8 data + 1 ACK) at the configured
bus frequency.  The caller supplies a safety multiplier and a minimum
floor so that each driver retains full control over its timing policy
without those values becoming public API.

Storing the result in adap->timeout makes it visible to all consumers of
that field, including the arbitration-loss retry loop in __i2c_transfer().

The function is gated by CONFIG_I2C_DYNAMIC_TIMEOUT.  When the config is
disabled, i2c_update_timeout() compiles to a no-op inline stub so drivers
that call it build cleanly and the existing static 1-second default is
preserved unchanged.

A timeout explicitly configured by userspace via the I2C_TIMEOUT ioctl is
stored in a new adap->user_timeout field and always takes precedence over
the kernel-computed value.  When userspace has not configured a timeout,
the computed value is used.  The ioctl keeps writing adap->timeout as well,
so adapters that never call i2c_update_timeout() continue to honour it
exactly as before.

As i2c_update_timeout() is an exported helper, guard against a zero bus
frequency from a misbehaving caller with WARN_ON_ONCE() and return early,
leaving the existing timeout untouched as a safe fallback rather than
dividing by zero.

Signed-off-by: Aniket Randive <aniket.randive@oss.qualcomm.com>
Reviewed-by: Mukesh Kumar Savaliya <mukesh.savaliya@oss.qualcomm.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://patch.msgid.link/20260911-master-v9-1-77ac458344e2@oss.qualcomm.com

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