isdn/capi: don't return NULL from capi_cmd2str()

capi_cmd2str() is used in many places to build log messages.
None of them is prepared to handle NULL as a result.
Change the function to return printable string "INVALID_COMMAND"
instead.

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