sf: unify erase commands

Analysis of the flash drivers shows that they all use 0x20 if the erase
size is 4KiB, or 0xd8 if it's larger.  So with this info in hand, we can
unify all the erase functionality in one place.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/drivers/mtd/spi/spi_flash.c b/drivers/mtd/spi/spi_flash.c
index f689cc4..4ab4e13 100644
--- a/drivers/mtd/spi/spi_flash.c
+++ b/drivers/mtd/spi/spi_flash.c
@@ -190,8 +190,7 @@
 		CMD_READ_STATUS, STATUS_WIP);
 }
 
-int spi_flash_cmd_erase(struct spi_flash *flash, u8 erase_cmd,
-			u32 offset, size_t len)
+int spi_flash_cmd_erase(struct spi_flash *flash, u32 offset, size_t len)
 {
 	u32 start, end, erase_size;
 	int ret;
@@ -209,7 +208,10 @@
 		return ret;
 	}
 
-	cmd[0] = erase_cmd;
+	if (erase_size == 4096)
+		cmd[0] = CMD_ERASE_4K;
+	else
+		cmd[0] = CMD_ERASE_64K;
 	start = offset;
 	end = start + len;