hwmon: (tmp108) Do not fail in I3C probe when I3C regmap is a module

I3C device probe fails when CONFIG_REGMAP_I3C=m:

	p3t1085_i3c 0-23615290090: error -ENODEV: Failed to register i3c regmap

Fix this by using the IS_ENABLED(CONFIG_REGMAP_I3C) macro in the code.

Fixes: c40655e3 ("hwmon: (tmp108) Add support for I3C device")
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Message-ID: <20241121121819.393104-1-jarkko.nikula@linux.intel.com>
[groeck: Remove #ifdef entirely; it is not needed due to dead code elimination]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>