wlcore: fixup an allocation

GFP_DMA isn't supposed to be used by itself.  This allocation is allowed
to sleep so it should be ORing it with GFP_KERNEL.
Also we should check for allocations errors.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>