mwifiex: avoid null-pointer-subtraction warning

clang complains about some NULL pointer arithmetic in this driver:

drivers/net/wireless/marvell/mwifiex/sta_tx.c:65:59: error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction]
        pad = ((void *)skb->data - (sizeof(*local_tx_pd) + hroom)-
                                                                 ^
drivers/net/wireless/marvell/mwifiex/uap_txrx.c:478:53: error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction]
        pad = ((void *)skb->data - (sizeof(*txpd) + hroom) - NULL) &

Rework that expression to do the same thing using a uintptr_t.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210927121656.940304-1-arnd@kernel.org

drivers/net/wireless/marvell/mwifiex/sta_tx.c

@@ -62,8 +62,8 @@ void *mwifiex_process_sta_txpd(struct mwifiex_private *priv,
 
 	pkt_type = mwifiex_is_skb_mgmt_frame(skb) ? PKT_TYPE_MGMT : 0;
 
-	pad = ((void *)skb->data - (sizeof(*local_tx_pd) + hroom)-
-			 NULL) & (MWIFIEX_DMA_ALIGN_SZ - 1);
+	pad = ((uintptr_t)skb->data - (sizeof(*local_tx_pd) + hroom)) &
+	       (MWIFIEX_DMA_ALIGN_SZ - 1);
 	skb_push(skb, sizeof(*local_tx_pd) + pad);
 
 	local_tx_pd = (struct txpd *) skb->data;

drivers/net/wireless/marvell/mwifiex/uap_txrx.c

@@ -475,8 +475,8 @@ void *mwifiex_process_uap_txpd(struct mwifiex_private *priv,
 
 	pkt_type = mwifiex_is_skb_mgmt_frame(skb) ? PKT_TYPE_MGMT : 0;
 
-	pad = ((void *)skb->data - (sizeof(*txpd) + hroom) - NULL) &
-			(MWIFIEX_DMA_ALIGN_SZ - 1);
+	pad = ((uintptr_t)skb->data - (sizeof(*txpd) + hroom)) &
+	       (MWIFIEX_DMA_ALIGN_SZ - 1);
 
 	skb_push(skb, sizeof(*txpd) + pad);