net: mvpp2: Use field just assigned in error test

net: mvpp2: Use field just assigned in error test

In mvpp2_probe the code attempts to get a value for "gop-port-id" and
assigns it to port->gop_id but it then tests port->id for being equal to
-1. That is an impossible test as port->id is a field of type u8 so
cannot be negative. Change the test to port->gop_id.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>