regulator: max8907: Properly set pmic pointer in max8907_regulator_remove()

Add missing platform_get_drvdata() call in max8907_regulator_remove(), this
fixes below build warning:

  CC [M]  drivers/regulator/max8907-regulator.o
drivers/regulator/max8907-regulator.c: In function 'max8907_regulator_remove':
drivers/regulator/max8907-regulator.c:353:23: warning: 'pmic' is used uninitialized in this function [-Wuninitialized]

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>