leds: blinkm: avoid uninitialized data use

gcc-8 reports missing error handling in blinkm_detect, when blinkm()
fails, tmpargs[] is uninitialized:

drivers/leds/leds-blinkm.c: In function 'blinkm_detect':
drivers/leds/leds-blinkm.c:555:6: error: 'tmpargs' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This adds a missing error checks.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>