ramips: use correct image size parameter for DIR-610 A1

Fixes the following error:

  bash: line 0: [: ralink_default_fw_size_4M: integer expression expected

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37829 3c298f89-4303-0410-b956-a3cf2f4a3e73