rtw88: use read_poll_timeout_atomic() for poll loop

This gives the added bonus of not wasting an extra udelay() if we're
timing out -- we double-check the register state one last time before
returning.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200821235733.1785009-1-briannorris@chromium.org