staging: mt7621-pci-phy: remove some unnecessary local variables

Device tree is not using child nodes anymore so the 'child_np' variable
can safely removed. This also simplifies the error path to be able to
directly return errors removing also the 'ret' variable.

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