Add workaround to skip wrong OMTU for basic rate

This patch is required until the MTU kernel patch is integrated. Without
it the kernel returns zero for basic rate OMTU.
diff --git a/attrib/gattrib.c b/attrib/gattrib.c
index 290cd96..8c15e52 100644
--- a/attrib/gattrib.c
+++ b/attrib/gattrib.c
@@ -409,7 +409,7 @@
 	if (bt_io_get(attrib->io, BT_IO_L2CAP, NULL,
 			BT_IO_OPT_OMTU, &omtu,
 			BT_IO_OPT_INVALID)) {
-		if (omtu > ATT_MAX_MTU)
+		if (omtu == 0 || omtu > ATT_MAX_MTU)
 			omtu = ATT_MAX_MTU;
 	} else
 		omtu = ATT_DEFAULT_LE_MTU;