ax88179_178a: Remove AX_MEDIUM_ALWAYS_ONE bit in AX_MEDIUM_STATUS_MODE register to avoid TX throttling

Remove AX_MEDIUM_ALWAYS_ONE in AX_MEDIUM_STATUS_MODE register.
Setting this bit may cause TX throttling in Half-Duplex mode.
Signed-off-by: Freddy Xin <freddy@asix.com.tw>
Signed-off-by: David S. Miller <davem@davemloft.net>