bnxt_en: map unsupported PAM4 speeds to nearest ifmedia types for ifconfig -m

[chandrakanth.patil_broadcom.com]

Summary

FreeBSD’s ifmedia API lacks enums for several PAM4 link modes (e.g., 50G-per-lane / PAM4_56 and 100G-per-lane / PAM4_112).
As a result, ifconfig -m displayed incorrect/unknown speeds.

This change reports the closest available ifmedia types for unsupported modes across CR (copper), SR (optical), LR, and
ER media, improving user-visible capabilities without affecting datapath behavior.

Examples:

PAM4_56 400G CR --> use IFM_400G_AUI8 (closest available)
PAM4_112 400G SR --> use IFM_400G_DR4 (closest available)

This is a display/workaround only, once FreeBSD adds native ifmedia constants for these modes, the mappings can be updated
or removed.