net/fm: call fm_port_to_index() with proper checks

Some of the fm_port_to_index() callers did not check for -1 return value and
used -1 as an array index.

Signed-off-by: Marian Rotariu <marian.rotariu@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>
diff --git a/drivers/net/fm/fm.h b/drivers/net/fm/fm.h
index 43de114..316e06e 100644
--- a/drivers/net/fm/fm.h
+++ b/drivers/net/fm/fm.h
@@ -143,6 +143,7 @@
 #define MAX_RXBUF_LOG2		11
 #define MAX_RXBUF_LEN		(1 << MAX_RXBUF_LOG2)
 
-#define PORT_IS_ENABLED(port)	fm_info[fm_port_to_index(port)].enabled
+#define PORT_IS_ENABLED(port)	(fm_port_to_index(port) == -1 ? \
+	0 : fm_info[fm_port_to_index(port)].enabled)
 
 #endif /* __FM_H__ */