m68k: Use sizeof_field() helper

Make use of the sizeof_field() helper instead of an open-coded version.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200527133942.GA10408@embeddedor


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>