net: atm: Add another IS_ENABLED(CONFIG_COMPAT) in atm_dev_ioctl

I removed compat's universal assignment to 0, which allows this if
statement to fall through when compat is passed with a value other
than 0.

Fixes: f9d19a74

 ("net: atm: Use IS_ENABLED in atm_dev_ioctl")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>