staging: r8188eu: remove unused field phead

The field phead in struct recv_buf is set but not used.
Remove it.

Signed-off-by: Abdun Nihaal <abdun.nihaal@gmail.com>
Link: https://lore.kernel.org/r/7ff76536af7478a00b300af9a6cb666e53d22aac.1646321515.git.abdun.nihaal@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>