mISDN: un-export symbol 'plx_lock'

'plx_lock' is only used within hfcmulti.c.

Fix this warning:

  drivers/isdn/hardware/mISDN/hfcmulti.c:176:1: warning: symbol 'plx_lock' shadows an earlier one
  drivers/isdn/hardware/mISDN/hfcmulti.c:175:19: originally declared here

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>