regulator: pwm: Prints error number along with detail

Prints the error number along with error message when any
error occurs. This help on getting the reason of failure
quickly from log without any code instrument.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>