ramips: Disable the SD driver by default on MT7621

ramips: Disable the SD driver by default on MT7621

The SD driver has been a long and dragging issue on the MT7621 platforms as it hasn't worked for
months resulting in unbootable devices. Until it's fixed, disable it in the default profiles.

Reports / Discussed here:

https://dev.openwrt.org/ticket/21392
https://dev.openwrt.org/ticket/21834
https://dev.openwrt.org/ticket/21995
https://forum.openwrt.org/viewtopic.php?id=59092

Signed-off-by: Daniel Engberg <daniel.engberg.lists@pyret.net>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49131 3c298f89-4303-0410-b956-a3cf2f4a3e73