staging: rtl8188eu: remove unnecessary declaration

The declaration of sta2sta_data_frame() is directly above
the function definition. Remove the unnecessary declaration.

This also clears a checkpatch issue.
CHECK: Lines should not end with a '('

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>