regulator: tps65090: Use IS_ERR to check return value of regulator_register

regulator_register never returns NULL.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked by: Venu Byravarasu <vbyravarasu@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>