ar71xx/mikrotik: re-enable 4KiB flash sector erase

The setting has been lost during the transition to 3.18.
The CONFIG_M25PXX_PREFER_SMALL_SECTOR_ERASE option is not
available anymore, so use CONFIG_MTD_SPI_NOR_USE_4K_SECTORS
instead.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 45161

target/linux/ar71xx/mikrotik/config-default

@@ -91,7 +91,6 @@ CONFIG_GPIO_74X164=y
 CONFIG_GPIO_LATCH=y
 # CONFIG_JFFS2_FS is not set
 CONFIG_LEDS_RB750=y
-CONFIG_M25PXX_PREFER_SMALL_SECTOR_ERASE=y
 CONFIG_MDIO_BITBANG=y
 CONFIG_MDIO_GPIO=y
 # CONFIG_MTD_CFI is not set
@@ -107,6 +106,7 @@ CONFIG_MTD_NAND_RB750=y
 CONFIG_MTD_NAND_RB91X=y
 # CONFIG_MTD_REDBOOT_PARTS is not set
 # CONFIG_MTD_SM_COMMON is not set
+CONFIG_MTD_SPI_NOR_USE_4K_SECTORS=y
 # CONFIG_MTD_TPLINK_PARTS is not set
 # CONFIG_OVERLAYFS_FS is not set
 CONFIG_RLE_DECOMPRESS=y