staging: octeon: check for pow0 before calling interface helper

Check for pow0 port first before calling the interface helper. This
avoids the following error log when setting up pow0 interface:

	cvmx_helper_get_interface_num: Illegal IPD port number

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>