wl12xx: enter psm only after station role was started

The station didn't get into psm after recovery, because
psm was configured before sta role was started.

Move wl1271_ps_set_mode() to be executed only after
the role was started.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>