rtl8xxxu: Check return value of kmemdup()

In rtl8xxxu_load_firmware() check the return value of kmemdup() and
error out with -ENOMEM in case of NULL to prevent a NULL pointer
dereference.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>