staging: r8188eu: remove xmitframe_direct function

xmitframe_direct is a simple wrapper around
rtw_xmitframe_coalesce and rtw_dump_xframe functions.
Many wrappers complicates code reading.

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