isdn: i4l: isdn_common: Fix set-but-unused variables.

The variable 'ch' is set but unused in isdn_capi_rec_hl_msg().  Just
kill it off.

Similarly for 'chidx' in isdn_ioctl() and 'di' in
isdn_capi_rec_hl_msg().

Signed-off-by: David S. Miller <davem@davemloft.net>