wl1251: Prepare for idle mode support

RFC for WL1251 idle mode support brought a few issues that are worth to
update before adding the idle mode support.

Since the idle mode can reuse the code that is now used in Power Save Mode
(PSM), the flag psm in struct wl1251 is changed to variable station_mode
to be able to distinguish between PSM and idle modes.

As the station mode is different than the power power save mode command
that is sent to chip, the enum wl1251_cmd_ps_mod values are used only when
communicating with the chip and new enum wl1251_station_mode values are used
inside the driver.

Confusing comment about psm and elp relation is removed since the PSM is
actually activated by putting the chip into Entreme Low Power (ELP) mode.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>