hamradio/mkiss: fix typo in compat_ioctl

My last commit introduced an typo causing the
compat_ioctl function to do nothing useful.
The obvious way for an ioctl function to work
is to look at the command, not the argument first.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>