staging: r8188eu: replace FIELD_OFFSET with offsetof

This driver defines FIELD_OFFSET which does the same as offsetof.
Replace this macro with offsetof.

Signed-off-by: Nam Cao <namcaov@gmail.com>
Link: https://lore.kernel.org/r/9443ef83f565eafe1e4f348412772d4ab966093a.1654994517.git.namcaov@gmail.com


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