regulator: tps65912: Remove unused define of TPS65912_MAX_REG_ID

Currently we define TPS65912_MAX_REG_ID as TPS65912_REG_LDO_10,
but TPS65912_REG_LDO_10 is not defined at all.
( It looks like a typo of TPS65912_REG_LDO10 )
Currently, TPS65912_MAX_REG_ID is not used in this driver,
it is safe to just remove it.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>