staging: rtl8723bs: remove useless variable rtw_AcceptAddbaReq

rtw_AcceptAddbaReq is a static variable, it is set once and never
modified. It is referenced only once, to assign its value to a member
of struct registry_priv.

Remove the variable, and move the meaningful part of the comment
near the declaration of the relevant field of struct registry_priv.

Signed-off-by: Aastha Gupta <aastha.gupta4104@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>