[posix-netif] lower transmit log level for OT_ERROR_DROP (#7275)

This commit helps reduce posix-netif WARN logs when transmitting
messages to Thread.

There are certain ICMPv6 messages (e.g. MLDv2) that OpenThread will
filter when transmitting, so the error `OT_ERROR_DROP` is expected.

This commit also assumes that OpenThread knows what it's doing when it
choose to drop a message, thus using lower log level for
`OT_ERROR_DROP`.

src/posix/platform/netif.cpp

@@ -847,7 +847,14 @@ exit:
 
     if (error != OT_ERROR_NONE)
     {
-        otLogWarnPlat("[netif] Failed to transmit, error:%s", otThreadErrorToString(error));
+        if (error == OT_ERROR_DROP)
+        {
+            otLogNotePlat("[netif] Message dropped by Thread", otThreadErrorToString(error));
+        }
+        else
+        {
+            otLogWarnPlat("[netif] Failed to transmit, error:%s", otThreadErrorToString(error));
+        }
     }
 }