staging: greybus: light: check delay_{on|off} before use

Even though we trust leds core that the pointers should be valid, we are
safer to check delay_{on|off} before use.

Also, this avoid a smatch warning:
drivers/staging/greybus/light.c:484 gb_blink_set()
warn: variable dereferenced before check 'delay_on' (see line 476)

Signed-off-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>