If the imp-controlled PHYs feature is enabled, driver can not
call phy driver interface to set loopback anymore and needs
to send command to firmware to start phy loopback.

Driver reuses the existing firmware command 0x0315 to start
phy loopback, just add a setting bit in this command. As this
command is not only for serdes loopback anymore, rename this
command to "xxx_COMMON_LOOPBACK", and modify function name,
macro name and logs related to it.

Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>