ar71xx: mark soft_config mtd part as writeable for RB-941-2nD

The soft_config partition must be writeable for rbcfg to be able to
enact changes to the routerboot configuration.
The read-only flag was a mistake in the initial patch. Removing it
brings mach-rb941.c in line with all other RB platforms.

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>

target/linux/ar71xx/files/arch/mips/ath79/mach-rb941.c

@@ -99,7 +99,6 @@ static struct mtd_partition rb941_spi_partitions[] = {
             .name = "soft_config",
             .offset = RB_SOFT_CFG_OFFSET,
             .size = RB_SOFT_CFG_SIZE,
-            .mask_flags = MTD_WRITEABLE,
     }, {
             .name = "rootfs",
             .offset = RB_ROOTFS_OFFSET,