regulator: pwm-regulator: Fix 'used uninitialized' warning

drivers/regulator/pwm-regulator.c:
  In function 'pwm_regulator_init_table':
drivers/regulator/pwm-regulator.c:172:14:
  warning: 'length' is used uninitialized in this function [-Wuninitialized]
    if ((length < sizeof(*duty_cycle_table)) ||
              ^
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>