wl12xx: add dynamic_ps_timeout debugfs file

Enable read/write of dynamic_ps_timeout which controls the timeout
of the dynamic PS implemented in the FW.
dynamic_ps_timeout is the timeout (in msec) until going back to PS
when there's no Rx/Tx

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