led: gpio: Fix possible ZERO_SIZE_PTR pointer dereferencing error.

Since we cannot make sure the 'pdata->num_leds' will always be none
zero here, and then if it equals to zero, the kmemdup() will return
ZERO_SIZE_PTR, which equals to ((void *)16).

So this patch fix this with just doing the zero check before calling
kmemdup().

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>