octeontx2-pf: cleanup transmit link deriving logic

Unlike OcteonTx2, the channel numbers used by CGX/RPM
and LBK on CN10K silicons aren't fixed in HW. They are
SW programmable, hence we cannot derive transmit link
from static channel numbers anymore. Get the same from
admin function via mailbox.

Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>