i3c: Add stub functions when I3C support is disabled

i3c: Add stub functions when I3C support is disabled

When I3C is disabled, unused functions are removed by the linker because
the driver relies on regmap and no I3C devices are registered, so normal
I3C paths are never called.

However, some drivers may still call low-level I3C transfer helpers.
Provide stub implementations to avoid adding conditional ifdefs everywhere.

Add stubs for i3c_device_do_xfers() and
i3c_device_get_supported_xfer_mode() only. Other stubs will be introduced
when they are actually needed.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512230418.nu3V6Yua-lkp@intel.com/
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20251230145718.4088694-1-Frank.Li@nxp.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>