amd-xgbe: fix MAC_AUTO_SW handling in CL37 AN

MAC_AUTO_SW (VR_MII_DIG_CTRL1 bit 9) enables automatic XPCS speed
mode switching after CL37 auto-negotiation and is only meaningful in
SGMII MAC mode. The original code unconditionally set this bit on
every call to xgbe_an37_set(), including when called from
xgbe_an37_disable() with enable=false. This left MAC_AUTO_SW=1 after
AN was disabled, causing the XPCS to autonomously switch speed from
stale AN state during subsequent mode changes, breaking SGMII speed
negotiation on 1G copper SFP modules.

Patrick: This was breaking negotiation for all 1G SFP modules,
not just copper modules.

Fixes: 42fd432fe6d3 ("amd-xgbe: align CL37 AN sequence as per databook")
Reported-by: Patrick Oppenlander <patrick.oppenlander@gmail.com>
Link: https://lore.kernel.org/netdev/CAEg67GmFS0Q4oSZkz8zWdOzckSth9_vBPiOy6a7-d697C2w2Xg@mail.gmail.com
Signed-off-by: Prashanth Kumar KR <PrashanthKumar.K.R@amd.com>
Tested-by: Patrick Oppenlander <patrick.oppenlander@gmail.com>
Link: https://patch.msgid.link/20260709095006.3683940-1-prashanthkumar.k.r@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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