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>

include/linux/mtd/mtd.h

@@ -48,8 +48,6 @@ struct erase_info {
 	uint64_t addr;
 	uint64_t len;
 	uint64_t fail_addr;
-	void (*callback) (struct erase_info *self);
-	u_long priv;
 	u_char state;
 };