staging: rtl8192e: Rename dm_bb_initialgain_backup

Use naming schema found in other rtlwifi devices.
Rename dm_bb_initialgain_backup to _rtl92e_dm_bb_initialgain_backup.

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