i2c: mpc: Remove CONFIG_PM_SLEEP ifdeffery

Use __maybe_unused for the suspend()/resume() hooks and get rid of
the CONFIG_PM_SLEEP ifdeffery to improve the code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Wolfram Sang <wsa@kernel.org>