regulator: pwm-regulator: Fix ' comparison between signed and unsigned integer' warning

drivers/regulator/pwm-regulator.c:
  In function ‘pwm_regulator_init_table’:
drivers/regulator/pwm-regulator.c:171:14:
  warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>