staging: r8188eu: Fix PPPoE tag insertion on little endian systems

In __nat25_add_pppoe_tag(), the tag length is read from the tag data
structure. The value is kept in network format, but read as raw value.
With -Warray-bounds, this results in the following gcc error/warning
when building the driver on alpha.

In function '__nat25_add_pppoe_tag',
    inlined from 'nat25_db_handle' at
	drivers/staging/r8188eu/core/rtw_br_ext.c:479:11:
arch/alpha/include/asm/string.h:22:16: error:
	'__builtin_memcpy' forming offset [40, 2051] is out of the bounds
	[0, 40] of object 'tag_buf' with type 'unsigned char[40]'

Add the missing be16_to_cpu() to fix the compile error. It should be
noted, however, that this fix means that the code did probably not work
on any little endian systems and/or that the driver has other endiannes
related issues. A build with C=1 suggests that this is indeed the case.
This patch does not attempt to fix any of those other issues.

Fixes: 15865124 ("staging: r8188eu: introduce new core dir for RTL8188eu driver")
Cc: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220404134338.3276991-1-linux@roeck-us.netSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/r8188eu/core/rtw_br_ext.c

@@ -70,7 +70,7 @@ static int __nat25_add_pppoe_tag(struct sk_buff *skb, struct pppoe_tag *tag)
 	struct pppoe_hdr *ph = (struct pppoe_hdr *)(skb->data + ETH_HLEN);
 	int data_len;
 
-	data_len = tag->tag_len + TAG_HDR_LEN;
+	data_len = be16_to_cpu(tag->tag_len) + TAG_HDR_LEN;
 	if (skb_tailroom(skb) < data_len)
 		return -1;