Staging: rtl8187se: Fix warning symbol should be static

This patch solves the sparse warning: "symbol 'ieee80211_ccmp_aes_encrypt' was
not declared. Should it be static?"

Signed-off-by: Andreea-Cristina Bernat <bernat.ada@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>