arm: sam9_l9260_defconfig: correct CONFIG_MTD_UBI_BEB_LIMIT

UBI has changed the MTD_UBI_BEB_LIMIT semantics. It used to be a percent of
total amount of eraseblock in the partition, and now it is the maximum
amount of bad eraseblocks on the entire devise per 1024 eraseblocks. So not
only the units changed, but also the meaning.

Richard Genoud <richard.genoud@gmail.com> says:

"I found the board:
https://www.olimex.com/dev/sam9-L9260.html
and the nand datasheet:
http://www.rockbox.org/wiki/pub/Main/LyrePrototype/K9xxG08UXM.pdf


page 11, we can see that the max_bad_bebper1024 is 25 (100 for 4096)"

Thus, use "25" for sam9.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>