Fix error checking in Vitesse IRQ config

phy_read() returns a negative number if there's an error, but the
error-checking code in the Vitesse driver's config_intr function
triggers if phy_read() returns non-zero.  Correct that.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>