isdn/gigaset: fix bas_gigaset AT read error handling

Rework the handling of USB errors in AT response reads
to fix a possible infinite retry loop and a memory leak,
and silence a few overly verbose kernel messages.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
CC: stable <stable@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>