octeontx2-af: poll for tx link credits before link mode change

commit 55e39e33ccd812b49cd897c166bff6d4887b1ae1 from
git@git.assembla.com:cavium/WindRiver.linux.git

During CGX/RPM link mode change while traffic is running, the link
state is disabled and subsequently the TX link credits are going to
negative value. Once the link is active again, the TX link credits
are not restored back leading to TX hang condition. This patch fixes
the issue by setting TL1 SW_XOFF before link mode change and polling
for TX credits to return back to original value.

Change-Id: I1da9767fbbc167ac28713670501673859a4bc3e1
Signed-off-by: Naveen Mamindlapalli <naveenm@marvell.com>
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/129011


Reviewed-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Tested-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: Bo Sun <Bo.Sun.CN@windriver.com>