staging: rtl8712: Replace FIELD_OFFSET() with offsetof()

Use the existing offsetof() macro instead of duplicating code.

Signed-off-by: Mauro Dreissig <mukadr@gmail.com>
Link: https://lore.kernel.org/r/20200701214420.5566-2-mukadr@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>