mwifiex: pcie: use posted write to wake up firmware

Depending on system factors (e.g., the PCIe link PM state), the first
read to wake up the Wifi firmware can take a long time. There is no
reason to use a (blocking, non-posted) read at this point, so let's just
use a write instead. Write vs. read doesn't matter functionality-wise --
it's just a dummy operation. But let's make sure to re-write with the
correct "ready" signature, since we check for that in other parts of the
driver.

This has been shown to decrease the time spent blocking in this function
on RK3399.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>