staging: r8188eu: prevent array underflow in rtw_hal_update_ra_mask()

The problem is that "mac_id" is a u32 so this check for underflow does
not work when "mac_id" is zero.  In that situation, "mac_id - 1" is
UINT_MAX instead of -1 so the condition is true.  It leads to an
array underflow on the next line.

Fixes: 8cd574e6 ("staging: r8188eu: introduce new hal dir for RTL8188eu driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210930122604.GB10068@kiliSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/r8188eu/hal/hal_intf.c

@@ -248,7 +248,7 @@ void rtw_hal_update_ra_mask(struct adapter *adapt, u32 mac_id, u8 rssi_level)
 #ifdef CONFIG_88EU_AP_MODE
 		struct sta_info *psta = NULL;
 		struct sta_priv *pstapriv = &adapt->stapriv;
-		if ((mac_id - 1) > 0)
+		if (mac_id >= 2)
 			psta = pstapriv->sta_aid[(mac_id - 1) - 1];
 		if (psta)
 			add_RATid(adapt, psta, 0);/* todo: based on rssi_level*/