staging: wfx: fix PS parameters when multiple vif are in use

staging: wfx: fix PS parameters when multiple vif are in use

When multiple vif are in use (= one access point and one station), and
when the channels are different, it is necessary to enable power save on
station.

The firmware check that steps are done in the correct order:
  - AP can't start if PS is not enable on the station
  - PS can't set on the station before the association has finished
    (= before the call set_bss_params)

Obviously, in add, when one of the interface disappears, it is necessary to
restore the power save status.

wfx_update_pm() is able to set the correct PS configuration. But it has
to be called at the right time:
   1. before hif_start(), but after the channel configuration is known
   2. after hif_set_bss_params()
   3. after hif_reset()

Therefore, the call to wfx_update_pm() from wfx_add_interface() is too
early to address 1.

The call after hif_set_bss_params() already exists.

For the symmetry, the call from wfx_remove_interface() (that handle 3.)
is also relocated.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200515083325.378539-15-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>