USB: serial: metro-usb: remove function prototypes

By rearranging the functions a bit, we can remove all function
prototypes.

Note, this also deleted the _close function, as it wasn't needed, it was
doing the same thing the cleanup function did, so just call that
instead.

Cc: Aleksey Babahin <tamerlan311@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>