staging: rtl8192u: r8192U_core: remove forward declarations in .c file

Checkpatch was giving a "externs should be avoided in .c files" because
of these forward declarations. As these were not useful in this case,
they have been removed.

Signed-off-by: Raphaël Beamonte <raphael.beamonte@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>