staging: rtl8723bs: replace EID_BSSCoexistence

Replace unique EID_BSSCoexistence constant with kernel provided
WLAN_EID_BSS_COEX_2040 from linux/ieee80211.h.

Signed-off-by: Ross Schmidt <ross.schm.dev@gmail.com>
Link: https://lore.kernel.org/r/20201208040733.379197-18-ross.schm.dev@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>