net: ipa: add IPA v5.1 and v5.5 to ipa_version_string()

Handle the case for v5.1 and v5.5 instead of returning "0.0".

Also reword the comment below since I don't see any evidence of such a
check happening, and - since 5.5 has been missing - can happen.

Fixes: 3aac8ec1 ("net: ipa: add some new IPA versions")
Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
Reviewed-by: Dawid Osuchowski <dawid.osuchowski@linux.intel.com>
Reviewed-by: Alex Elder <elder@riscstar.com>
Link: https://patch.msgid.link/20250728-ipa-5-1-5-5-version_string-v1-1-d7a5623d7ece@fairphone.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>