staging: wfx: fix pm_mode timeout

Maximum request time (how long a request wait for the medium) is set in
firmware to 512TU

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191217161318.31402-37-Jerome.Pouiller@silabs.com


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