regulator: Fix an s5m8767 build failure

Due to a merge conflict we are getting this:

drivers/regulator/s5m8767.c: In function ‘s5m8767_pmic_probe’:
drivers/regulator/s5m8767.c:575:2: error: implicit declaration of function
‘s5m_reg_write’ [-Werror=implicit-function-declaration]

This is fixed by fully converting this driver to the new s5m API.

Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>