regulator: twl: fix use of integer as pointer

The TWL6030_FIXED_LDO() macro passes the TWL_FIXED_LDO()
macro an 0x0 instead of a NULL. Changing this to a NULL
fixes the following warnings:

drivers/regulator/twl-regulator.c:1068:1: warning: Using plain integer as NULL pointer
drivers/regulator/twl-regulator.c:1069:1: warning: Using plain integer as NULL pointer
drivers/regulator/twl-regulator.c:1070:1: warning: Using plain integer as NULL pointer
drivers/regulator/twl-regulator.c:1071:1: warning: Using plain integer as NULL pointer
drivers/regulator/twl-regulator.c:1072:1: warning: Using plain integer as NULL pointer
drivers/regulator/twl-regulator.c:1073:1: warning: Using plain integer as NULL pointer

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>