mtd: nand: refactor erase_cmd() to return chip status

The nand_chip::erase_cmd callback previously served a dual purpose; for
one, it allowed a per-flash-chip override, so that AG-AND devices could
use a different erase command than other NAND. These AND devices were
dropped in commit 14c6578683 (mtd: nand:
remove AG-AND support). On the other hand, some drivers (denali and
doc-g4) need to use this sort of callback to implement
controller-specific erase operations.

To make the latter operation easier for some drivers (e.g., ST's new BCH
NAND driver), it helps if the command dispatch and wait functions can be
lumped together, rather than called separately.

This patch does two things:
 1. Pull the call to chip->waitfunc() into chip->erase_cmd(), and return
    the status from this callback
 2. Rename erase_cmd() to just erase(), since this callback does a
    little more than just send a command

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Tested-by: Lee Jones <lee.jones@linaro.org>

include/linux/mtd/nand.h

@@ -552,8 +552,7 @@ struct nand_buffers {
  * @ecc:		[BOARDSPECIFIC] ECC control structure
  * @buffers:		buffer structure for read/write
  * @hwcontrol:		platform-specific hardware control structure
- * @erase_cmd:		[INTERN] erase command write function, selectable due
- *			to AND support.
+ * @erase:		[REPLACEABLE] erase function
  * @scan_bbt:		[REPLACEABLE] function to scan bad block table
  * @chip_delay:		[BOARDSPECIFIC] chip dependent delay for transferring
  *			data from array to read regs (tR).
@@ -637,7 +636,7 @@ struct nand_chip {
 	void (*cmdfunc)(struct mtd_info *mtd, unsigned command, int column,
 			int page_addr);
 	int(*waitfunc)(struct mtd_info *mtd, struct nand_chip *this);
-	void (*erase_cmd)(struct mtd_info *mtd, int page);
+	int (*erase)(struct mtd_info *mtd, int page);
 	int (*scan_bbt)(struct mtd_info *mtd);
 	int (*errstat)(struct mtd_info *mtd, struct nand_chip *this, int state,
 			int status, int page);