wcn36xx: Mark internal smd functions static

This commit marks all smd.c functions that are only used inside of smd.c as
static. Previous commits added some VHT specific setup functions non-static
which is the right thing to do in terms of having granular git commits that
compile warning free. What we really want is for local not global scope on
those functions.

This patch makes the conversion from global to local scope.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200910150845.2179320-2-bryan.odonoghue@linaro.org