fixes: 81737c1 mtd: Fix erasing of devices >4GiB

Several places erased a complete flash partition passing ~0 as count to
erase(). With the above commit count to erase was changed from an unsigned
type to a signed type, so the (count > f->size - offset) check in
erase() no longer triggers and the ~0 count is no longer adjusted to the whole
device size. Among other things this results in saveenv failures on NOR
flashes.
This patch fixes this by introducing an explicit macro for erasing the
whole device which is tested for in erase(). All other negative values
are rejected.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reported-by: Giorgio <giorgio.nicole@arcor.de>