net: mediatek: use regmap api to modify ethsys registers

The address returned by regmap_get_range() is not remapped. Directly r/w
to this address is ok for ARM platforms since it's idential to the virtual
address.

But for MIPS platform only virtual address should be used for access.
To solve this issue, the regmap api regmap_read/regmap_write should be used
since they will remap address before accessing.

Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>