mtd: Stop assuming mtd_erase() is asynchronous

None of the mtd->_erase() implementations work in an asynchronous manner,
so let's simplify MTD users that call mtd_erase(). All they need to do
is check the value returned by mtd_erase() and assume that != 0 means
failure.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Richard Weinberger <richard@nod.at>