staging: rtl8723bs: use standard offsetof in cfg80211 operations

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 <knavaneeth786@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://patch.msgid.link/20251125112059.16913-3-knavaneeth786@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>