wifi: airo: do not assign -1 to unsigned char

With char becoming unsigned by default, and with `char` alone being
ambiguous and based on architecture, we get a warning when assigning the
unchecked output of hex_to_bin() to that unsigned char. Mark `key` as a
`u8`, which matches the struct's type, and then check each call to
hex_to_bin() before casting.

Cc: Kalle Valo <kvalo@kernel.org>
Cc: linux-wireless@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221024162843.535921-1-Jason@zx2c4.com

drivers/net/wireless/cisco/airo.c

@@ -5232,7 +5232,7 @@ static int get_wep_tx_idx(struct airo_info *ai)
 	return -1;
 }
 
-static int set_wep_key(struct airo_info *ai, u16 index, const char *key,
+static int set_wep_key(struct airo_info *ai, u16 index, const u8 *key,
 		       u16 keylen, int perm, int lock)
 {
 	static const unsigned char macaddr[ETH_ALEN] = { 0x01, 0, 0, 0, 0, 0 };
@@ -5283,7 +5283,7 @@ static void proc_wepkey_on_close(struct inode *inode, struct file *file)
 	struct net_device *dev = pde_data(inode);
 	struct airo_info *ai = dev->ml_priv;
 	int i, rc;
-	char key[16];
+	u8 key[16];
 	u16 index = 0;
 	int j = 0;
 
@@ -5311,12 +5311,22 @@ static void proc_wepkey_on_close(struct inode *inode, struct file *file)
 	}
 
 	for (i = 0; i < 16*3 && data->wbuffer[i+j]; i++) {
+		int val;
+
+		if (i % 3 == 2)
+			continue;
+
+		val = hex_to_bin(data->wbuffer[i+j]);
+		if (val < 0) {
+			airo_print_err(ai->dev->name, "WebKey passed invalid key hex");
+			return;
+		}
 		switch(i%3) {
 		case 0:
-			key[i/3] = hex_to_bin(data->wbuffer[i+j])<<4;
+			key[i/3] = (u8)val << 4;
 			break;
 		case 1:
-			key[i/3] |= hex_to_bin(data->wbuffer[i+j]);
+			key[i/3] |= (u8)val;
 			break;
 		}
 	}