i2c: designware: Fix build error when !CONFIG_PM_SLEEP

Commit ("i2c: designware: Rename platform driver probe and PM
functions") introduced "'dw_i2c_plat_prepare' undeclared here" and
"'dw_i2c_plat_complete' undeclared here" build errors when
CONFIG_PM_SLEEP is not set.

Fix this by renaming NULL defined dw_i2c_prepare and dw_i2c_complete PM
hooks to dw_i2c_plat_prepare and dw_i2c_plat_complete since this was
obviously missing from the commit.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>