mtd: allow partial block unlock

mtd: allow partial block unlock

This allows sysupgrade for devices such as the Gateworks Avila/Cambria
product families based on the ixp4xx using the redboot bootloader with
combined FIS directory and RedBoot config partitions on larger FLASH
devices with larger eraseblocks.

This second iteration of this patch addresses previous issues:
  - whitespace breakage fixed
  - unlock in all scenarios
  - simplification and fix logic bug

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@33681 3c298f89-4303-0410-b956-a3cf2f4a3e73