wl12xx: fix a couple of sparse warnings about undeclared functions

Fix the following sparse warnings:

drivers/net/wireless/wl12xx/main.c:1129:5: warning: symbol '__wl1271_plt_stop' was not declared. Should it be static?
drivers/net/wireless/wl12xx/main.c:2988:5: warning: symbol 'wl1271_op_ampdu_action' was not declared. Should it be static?

Both functions should be static.

Signed-off-by: Luciano Coelho <coelho@ti.com>