bnxt: avoid unintialized use of media_type in bnxt_add_media_types()

[gallatin]

Summary

When 5e6e4f752833 landed, it inadvertently allowed unsupported / unknown
phys with a valid link_info->sig_mode to use media_type uninitialized.
Fix this by returning from those cases, so as to avoid using media_type
uninitialized.

Pointed out by imp (forwarding an llm's post-commit review of previous changes to this file)

Test Plan

Tested by commenting out support for FR4 optics and observing no regressions.