mmc_write.c: Make mmc_berase do 32bit safe 64bit math

We want to see if the requested start or total block count are
unaligned.  We discard the whole numbers and only care about the
remainder.  Update the code to use div_u64_rem here and add a comment.

Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Cc: Bernhard Nortmann <bernhard.nortmann@web.de>
Reported-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Tested-by: Bernhard Nortmann <bernhard.nortmann@web.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>