staging: mt7621-pci: add phy exit call if phy_power_on call fails

Add missing call to 'phy_exit' function if the phy_power_on call fails.
With this call added the error path is properly handled.

Fixes: 07420a02 ("staging: mt7621-pci: use gpio perst instead of builtin behaviour")
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>