staging: rtl8192e: Merge/Remove _rtl92e_dm_bb_initialgain_backup

Merge rtl92e_dm_backup_state and _rtl92e_dm_bb_initialgain_backup.
Static function is called only once and dm_backup_state had short body.

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>