net: ethernet: ti: am65-cpsw: Move mode specific config to mac_config()

Move the interface mode specific configuration to the mac_config()
callback am65_cpsw_nuss_mac_config().

Also, do not reset the MAC Control register on mac_link_down(). Only
clear those bits that can possibly be set in mac_link_up().

Let the MAC remain in IDLE state after mac_link_down(). Bring it out of
the IDLE state on mac_link_up().

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>