wlcore: Use spin_trylock in wlcore_irq() to see if we need to queue tx

We currently have a collection of flags and locking between the
threaded irq and tx work:

- wl->flags bitops
- wl->mutex
- wl->wl_lock spinlock

The bitops flags do not need a spinlock around them, and we only need
the spinlock to see if we need to queue tx work or not. And wlcore_irq()
holds the mutex.

To simplify the locking, we can use spin_trylock and always queue tx
work unless we know there's nothing to do.

Let's also update the comment a bit while at it.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200702162951.45392-4-tony@atomide.com