staging: r8188eu: refactor recvframe_defrag()

Local variables pfhdr and prframe always have a same value,
so one of them is unnecessary.

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>