mISDN/hfcsusb: remove unused variable hfcsusb_rev

Removed unused variable hfcsusb_rev (-Wunused-variable) and
copied Revision info to file comment block above to keep the info.

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: David S. Miller <davem@davemloft.net>