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/include/net.h b/include/net.h
index a180881..ab571eb 100644
--- a/include/net.h
+++ b/include/net.h
@@ -117,7 +117,7 @@
 extern void eth_set_current(void);		/* set nterface to ethcur var */
 #endif
 extern struct eth_device *eth_get_dev(void);	/* get the current device MAC */
-extern struct eth_device *eth_get_dev_by_name(char *devname); /* get device */
+extern struct eth_device *eth_get_dev_by_name(const char *devname);
 extern struct eth_device *eth_get_dev_by_index(int index); /* get dev @ index */
 extern int eth_get_dev_index (void);		/* get the device index */
 extern void eth_parse_enetaddr(const char *addr, uchar *enetaddr);