net: stmmac: dwmac-mediatek: fix wrong delay value issue when resume back

mac_delay value will be divided by 550/170 in mt2712_delay_ps2stage(),
which is invoked at the beginning of mt2712_set_delay(), and the value
should be restored at the end of mt2712_set_delay().
Or, mac_delay will be divided again when invoking mt2712_set_delay()
when resume back.
So, add mt2712_delay_stage2ps() to mt2712_set_delay() to recovery the
original mac_delay value.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>