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

This driver has dependencies on mt7621-gpio and mt7621-pci-phy which
are init in later stages. Hence, when this driver is probed it is always
returning 'EPROBE_DEFER' and being initialized afterwards. Use function
'module_init' to just initialize later.

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