isdn/gigaset: drop debug check on isochronous write

With CONFIG_GIGASET_DEBUG set, every isochronous USB frame after
an erroneous one was checked for more errors. This produced only
noise messages in practice, so drop it.

Impact: cleanup
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>