isdn/capi: drop two dead if branches

The last branch in command_2_index() cannot be reached since
c==0xff is already caught by the first "if".
The empty second branch makes no difference since no other branch
will be taken for c<0x0f.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>