mISDN: timerdev: use __user for mISDN_read's buffer argument

Fix this warning:

  drivers/isdn/mISDN/timerdev.c:264:11: warning: incorrect type in initializer (incompatible argument 2 (different address spaces))
  drivers/isdn/mISDN/timerdev.c:264:11:    expected int ( *read )( ... )
  drivers/isdn/mISDN/timerdev.c:264:11:    got int ( static [toplevel] *<noident> )( ... )

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