staging: mt7621-pci-phy: use 'module_init' instead of 'arch_initcall'

Init driver as 'arch_initcall()' does not work. It causes phy_create()
to be called before the phy module is initialized, so 'phy_class' is NULL,
the new phy isn't placed in the right class, and it cannot be found.
Change to 'module_init()' which works properly in this case.

Fixes: 00981d31: staging: mt7621-pci-phy: add new driver for phy part of mt7621-pci
Reported-by: NeilBrown <neil@brown.name>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>