leds: leds-lp5562 allow firmware files up to the maximum length

Firmware files are in ASCII, using 2 hex characters per byte. The
maximum length of a firmware string is therefore

16 (commands) * 2 (bytes per command) * 2 (characters per byte) = 64

Fixes: ff45262a ("leds: add new LP5562 LED driver")
Signed-off-by: Nick Stoughton <nstoughton@logitech.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>