staging: rtl8723bs: use standard offsetof in cfg80211 operations

Replace usage of the custom FIELD_OFFSET macro with the standard
offsetof() macro in ioctl_cfg80211.c. This improves code readability
and uses the kernel's standard mechanism.

Also include <linux/stddef.h> in basic_types.h to ensure offsetof()
is available for this and future conversions.

Signed-off-by: Navaneeth K <[e-mail]>
Reviewed-by: Dan Carpenter <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/rtl8723bs/include/basic_types.h

···
 #define FAIL	(-1)
 
 #include <linux/types.h>
+#include <linux/stddef.h>
 
 #define FIELD_OFFSET(s, field)	((__kernel_ssize_t)&((s *)(0))->field)
 

drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c

···
 
 		if (wep_key_len > 0) {
 			wep_key_len = wep_key_len <= 5 ? 5 : 13;
-			wep_total_len = wep_key_len + FIELD_OFFSET(struct ndis_802_11_wep, key_material);
+			wep_total_len = wep_key_len +
+				offsetof(struct ndis_802_11_wep, key_material);
 			pwep = rtw_malloc(wep_total_len);
 			if (!pwep) {
 				ret = -ENOMEM;