staging: rtl8188eu: refactor rtl88eu_dm_update_rx_idle_ant()

Refactor rtl88eu_dm_update_rx_idle_ant() to reduce indentation level
and clear line over 80 characters checkpatch warnings.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20200105194936.5477-2-straube.linux@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>