isdn/gigaset: improve error handling querying firmware version

commit e055d03dc088a990fe5ea24a2d64033a168da23c upstream.

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>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: adjust indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>