staging: rtl8712: remove enum WIFI_FRAME_SUBTYPE

The values defined in enum WIFI_FRAME_SUBTYPE are the same the #define
IEEE80211_STYPE_xxx from <linux/ieee80211.h>

Use theses values to avoid code duplication.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/5a6fdcf0b5eb43c3d5511b5badd60bfac9389628.1617911201.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>