staging: mt7621-pci: replace return value if devm_pci_alloc_host_bridge call fails

Driver probe function calls 'devm_pci_alloc_host_bridge'. If this call fails
it is returning -ENODEV. Return -ENOMEM instead which is more accurate for
this.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>