leds: pca955x: Prevent crippled LED device name

leds: pca955x: Prevent crippled LED device name

In case platform data provided empty LED name string the resulting
LED class device name would be crippled. Use corresponding LED chip
bit in place of "function" segment of LED class device name then to
make the LEDs at least distinguishable.

Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Reported-by: Colin King <colin.king@canonical.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Nate Case <ncase@xes-inc.com>