staging: wfx: split wfx_get_ps_timeout() from wfx_update_pm()

In the next commit, we will have to compute the PS timeout without
changing the power save status of the device. This patch introduces
wfx_get_ps_timeout() for that job and make wfx_update_pm() relies on it.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200526171821.934581-7-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>