regulator: lp8755: Remove enum bucks

We already have enum lp8755_bucks in lp8755.h, so it looks pointless adding
enum bucks in lp8755.c.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Tested-by: Daniel Jeong <gshark.jeong@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>