regulator: change debug statement be consistent with the style of the rest

change the "mc13783_regulator_probe" string in the function
"mc13783_regulator_probe" to "__func__" for the debug statement be consistent
with the style of the rest of the file.

Signed-off-by: Wanlong Gao <wanlong.gao@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>