wcn36xx: do not overwite CROSS_COMPILE

Do not overwrite CROSS_COMPILE so that another cross compiler can be
utilized (other than the one provided by Android / CyanogenMod).

Signed-off-by: Jason Mobarak <jam@cozybit.com>
Merge pull request #119 from silverjam/dont-overwite-cross-compiler

wcn36xx: don't overwite CROSS_COMPILE