drivers/isdn/capi: fix sparse warning: context imbalance

Impact: Attribute functions with __acquires(...) resp. __releases(...).

Fix this sparse warnings:
  drivers/isdn/capi/kcapi_proc.c:241:13: warning: context imbalance in 'capi_driver_start' - wrong count at exit
  drivers/isdn/capi/kcapi_proc.c:252:13: warning: context imbalance in 'capi_driver_stop' - unexpected unlock

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