isdn/gigaset: remove dummy CAPI method implementations

Dummy implementations for the optional CAPI controller operations
load_firmware and reset_ctr can cause userspace callers to hang
indefinitely. It's better not to implement them at all.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Acked-by: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: David S. Miller <davem@davemloft.net>