platform: refactor wifi_utils_is_wifi() not to pass sysfs_path

wifi_utils_is_wifi() only has one caller, so it's very clear
what the passed in @sysfs_path contains. Instead of accepting
a redundant argument, compute the sysfs path internally based
on @iface alone.

src/platform/nm-linux-platform.c

@@ -736,7 +736,7 @@ _linktype_get_type (NMPlatform *platform,
 		}
 
 		/* Fallback for drivers that don't call SET_NETDEV_DEVTYPE() */
-		if (wifi_utils_is_wifi (ifname, sysfs_path))
+		if (wifi_utils_is_wifi (ifname))
 			return NM_LINK_TYPE_WIFI;
 
 		if (arptype == ARPHRD_ETHER) {