rtw88: pci: use macros to access PCI DBI/MDIO registers

Add some register and bit macros to access DBI/MDIO register. This
should not change the logic.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>