Fix compile warnings for const correctness

Commit 6e37b1a3a25004d3df5867de49fff6b3fc9c4f04 modifies several net calls
to take a (const char *) parameter instead of (char *), but in some cases
the modified functions call other functions taking (char *).  The end result
is warnings about discarding the const qualifier.

This patch fixes these other function signatures.

Signed-off-by: Ben Warren <biggerbadderben@gmail.com>
diff --git a/drivers/net/at91_emac.c b/drivers/net/at91_emac.c
index 0912f52..d82459b 100644
--- a/drivers/net/at91_emac.c
+++ b/drivers/net/at91_emac.c
@@ -162,7 +162,7 @@
 
 #if defined(CONFIG_MII) || defined(CONFIG_CMD_MII)
 
-at91_emac_t *get_emacbase_by_name(char *devname)
+at91_emac_t *get_emacbase_by_name(const char *devname)
 {
 	struct eth_device *netdev;