staging:r8188eu: Remove struct pkt_file from rtw_xmitframe_coalesce()

Struct pkt_file is a base to simple wrapper for skb_copy_bits().
Eliminate struct pkt_file usage in rtw_xmitframe_coalesce().

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>