net: miiphybb: fix casting error

Since the return value is a signed int, if the leading MSB of rdreg is a 1,
it will get signed extended and will return a negative value which is an
error even though we read the correct value.

Fixes: dfcc496ed7 ("net: mii: Changes not made by spatch")
Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Acked-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

drivers/net/phy/miiphybb.c

@@ -232,7 +232,7 @@ static void miiphy_pre(struct bb_miiphy_bus *bus, char read,
  */
 int bb_miiphy_read(struct mii_dev *miidev, int addr, int devad, int reg)
 {
-	short rdreg; /* register working value */
+	unsigned short rdreg; /* register working value */
 	int v;
 	int j; /* counter */
 	struct bb_miiphy_bus *bus;