net: phylink: treat PSGMII as an inband capable interface

PSGMII (the Qualcomm 5-port SGMII) conveys the link negotiation result
from the PHY back to the MAC through per-channel in-band SGMII words,
exactly like SGMII and QSGMII.

However, PHY_INTERFACE_MODE_PSGMII is missing from
phylink_get_inband_type(), so phylink reports INBAND_NONE for it and
phylink_pcs_neg_mode() falls back to PHYLINK_PCS_NEG_NONE. The PCS is
then programmed in force mode and its control-register speed bits (which
default to 1000base) are used, so a slower copper link - e.g. 100base-T
- is reported as 1Gbps and cannot pass traffic.

Classify PSGMII alongside SGMII and QSGMII as INBAND_CISCO_SGMII so the
PCS negotiates in-band and the resolved link speed comes from the PHY
in-band word.

Also add PSGMII to the generic clause 22 PCS helper functions which
handle the SGMII in-band word. Without this, a PCS using these helpers
would still fall through to the default handling and force the link
state to false in phylink_mii_c22_pcs_decode_state(), fail to encode
the SGMII advertisement, and get rejected by phylink_get_link_timer_ns().

Signed-off-by: Sandeep Sondagar <sandeepsondagar@gmail.com>
Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20260809-phylink-psgmii-v3-1-908dcd3a9e3d@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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