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>

drivers/isdn/hardware/mISDN/hfcmulti.c

@@ -173,7 +173,6 @@ static int interrupt_registered;
 static struct hfc_multi *syncmaster;
 static int plxsd_master; /* if we have a master card (yet) */
 static spinlock_t plx_lock; /* may not acquire other lock inside */
-EXPORT_SYMBOL(plx_lock);
 
 #define	TYP_E1		1
 #define	TYP_4S		4