staging: rtl8192u: propagate errors in write_nic_dword

Propagate errors from kzalloc and usb_control_msg and change the
return type of write_nic_dword from void to int.

Signed-off-by: Salah Triki <salah.triki@acm.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>