drivers/isdn/mISDN: fix sparse warnings: make symbols static

Fix this sparse warnings:
  drivers/isdn/mISDN/clock.c:44:1: warning: symbol 'iclock_lock' was not declared. Should it be static?
  drivers/isdn/mISDN/clock.c:45:9: warning: symbol 'iclock_count' was not declared. Should it be static?
  drivers/isdn/mISDN/clock.c:46:17: warning: symbol 'iclock_tv' was not declared. Should it be static?
  drivers/isdn/mISDN/clock.c:47:9: warning: symbol 'iclock_tv_valid' was not declared. Should it be static?
  drivers/isdn/mISDN/clock.c:48:20: warning: symbol 'iclock_current' was not declared. Should it be static?

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>