drivers/isdn/act2000: fix sparse warning: symbol shadows an earlier one

Impact: Rename local variable 'c' to 't'.

Fix this sparse warning:
  drivers/isdn/act2000/act2000_isa.c:260:70: warning: symbol 'c' shadows an earlier one

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