staging: rtl8723au: c2h_wk_callback(): Use GFP_KERNEL, and check return value

It is safe to use GFP_KERNEL in a workqueue handler, and check the
return value. If the malloc fails, skip and try again.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>