leds: lp8860: Remove duplicate NULL checks for gpio_desc

gpiod_*() API check already for the NULL, no need
to repeat that in the driver.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230215170403.84449-2-andriy.shevchenko@linux.intel.com