staging: rtl8192e: Rename dm_initial_gain

Use naming schema found in other rtlwifi devices.
Rename dm_initial_gain to _rtl92e_dm_initial_gain.

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