hwmon: (pmbus/ibm-cffps) Switch LEDs to blocking brightness call

Since i2c_smbus functions can sleep, the brightness setting function
for this driver must be the blocking version to avoid scheduling while
atomic.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20191106200106.29519-2-eajames@linux.ibm.com
Fixes: ef9e1cdf

 ("hwmon: (pmbus/cffps) Add led class device for power supply fault led")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>