net: ethernet: mediatek: refactoring mtk_hw_init to be reused

the existing mtk_hw_init includes hardware and software
initialization inside so that it is slightly hard to reuse
them for the process of the reset recovery, so some splitting
is made here for keeping hardware initializing relevant thing
and the else such as IRQ registration and MDIO initialization
what are all about to the interface of core driver moved to the
other proper place because they have no needs to register IRQ and
re-initialize structure again during the reset process.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>