i3c: fix module_i3c_i2c_driver() with I3C=n

i3c: fix module_i3c_i2c_driver() with I3C=n

When CONFIG_I3C is disabled and the i3c_i2c_driver_register() happens
to not be inlined, any driver calling it still references the i3c_driver
instance, which then causes a link failure:

x86_64-linux-ld: drivers/hwmon/lm75.o: in function `lm75_i3c_reg_read':
lm75.c:(.text+0xc61): undefined reference to `i3cdev_to_dev'
x86_64-linux-ld: lm75.c:(.text+0xd25): undefined reference to `i3c_device_do_priv_xfers'
x86_64-linux-ld: lm75.c:(.text+0xdd8): undefined reference to `i3c_device_do_priv_xfers'

This issue was part of the original i3c code, but only now caused problems
when i3c support got added to lm75.

Change the 'inline' annotations in the header to '__always_inline' to
ensure that the dead-code-elimination pass in the compiler can optimize
it out as intended.

Fixes: 6071d10413ff ("hwmon: (lm75) add I3C support for P3T1755")
Fixes: 3a379bbcea0a ("i3c: Add core I3C infrastructure")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/r/20250725090609.2456262-1-arnd@kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>