staging:rtl8821ae:Fix sparse warning of static function declaration

This patch makes the function _rtl8821ae_map_hwqueue_to_fwqueue
static as it is used only in the file trx.c.

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>