ar71xx/mikrotik: enable small sector erase in the m25p80 driver

The serial flash devices used on the ROuterBOARDs are
supporting 4KiB erase blocks. Enable the small sector
erase option in the m25p80 driver to avoid superfluous
erase/write of adjacent blocks.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39696 3c298f89-4303-0410-b956-a3cf2f4a3e73