configs/{ea3250, fdi3250, phy3250}: lock down kernel headers version

Update {ea3250,fdi3250,phy3250}_defconfig to lock down kernel headers to
an appropiate version.
Otherwise the target is building with, at the moment of this writing,
version 3.7 headers and a 2.6.34 kernel which usually isn't wise.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>