rt2800: do not crash if spec->channels is NULL

In case the spec->channels was not specified, print warning instead
of hard crash the kernel.

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Tested-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>