isdn/gigaset: improve error handling querying firmware version

An out-of-place "OK" response to the "AT+GMR" (get firmware version)
command turns out to be, more often than not, a delayed response to
a previous command rather than an actual error, so continue waiting
for the version number in that case.

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