regulator: lp8788-buck: Silence build warning

This driver use id as array index, thus add bounder checking for id.

This patch fixes below build warning:
drivers/regulator/lp8788-buck.c: In function 'lp8788_buck_probe':
drivers/regulator/lp8788-buck.c:490:8: warning: array subscript is above array bounds [-Warray-bounds]
drivers/regulator/lp8788-buck.c:489:63: warning: array subscript is above array bounds [-Warray-bounds]

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>