mt76: fix monitor mode crash with sdio driver

mt7921s driver may receive frames with fragment buffers. If there is a
CTS packet received in monitor mode, the payload is 10 bytes only and
need 6 bytes header padding after RXD buffer. However, only RXD in the
first linear buffer, if we pull buffer size RXD-size+6 bytes with
skb_pull(), that would trigger "BUG_ON(skb->len < skb->data_len)" in
__skb_pull().

To avoid the nonlinear buffer issue, enlarge the RXD size from 128 to
256 to make sure all MCU operation in linear buffer.

[   52.007562] kernel BUG at include/linux/skbuff.h:2313!
[   52.007578] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[   52.007987] pc : skb_pull+0x48/0x4c
[   52.008015] lr : mt7921_queue_rx_skb+0x494/0x890 [mt7921_common]
[   52.008361] Call trace:
[   52.008377]  skb_pull+0x48/0x4c
[   52.008400]  mt76s_net_worker+0x134/0x1b0 [mt76_sdio 35339a92c6eb7d4bbcc806a1d22f56365565135c]
[   52.008431]  __mt76_worker_fn+0xe8/0x170 [mt76 ef716597d11a77150bc07e3fdd68eeb0f9b56917]
[   52.008449]  kthread+0x148/0x3ac
[   52.008466]  ret_from_fork+0x10/0x30
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt76.h

@@ -19,7 +19,7 @@
 
 #define MT_MCU_RING_SIZE	32
 #define MT_RX_BUF_SIZE		2048
-#define MT_SKB_HEAD_LEN		128
+#define MT_SKB_HEAD_LEN		256
 
 #define MT_MAX_NON_AQL_PKT	16
 #define MT_TXQ_FREE_THR		32