isdn: isdnloop: fix pointer dereference bug

clang has spotted an ancient code bug and warns about it with:

drivers/isdn/isdnloop/isdnloop.c:573:12: error: address of array 'card->rcard' will always evaluate to 'true' [-Werror,-Wpointer-bool-conversion]

This is an array of pointers, so we should check if a specific
pointer exists in the array before using it, not whether the
array itself exists.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>