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: 3aac8ec1c028 ("net: ipa: add some new IPA versions")
Signed-off-by: Luca Weiss <[e-mail]>
Reviewed-by: Dawid Osuchowski <[e-mail]>
Reviewed-by: Alex Elder <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ipa/ipa_sysfs.c

···
 		return "4.11";
 	case IPA_VERSION_5_0:
 		return "5.0";
+	case IPA_VERSION_5_1:
+		return "5.1";
+	case IPA_VERSION_5_5:
+		return "5.5";
 	default:
-		return "0.0";	/* Won't happen (checked at probe time) */
+		return "0.0";	/* Should not happen */
 	}
 }