USB: serial: metro-usb: remove function header comments

They aren't needed, make the checkpatch tool unhappy, and in some
places, aren't even correct.  So just remove them, they get in the way
and are messy.

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