lan743x: replace polling loop by wait_event_timeout()

The driver's ISR sends a 'software interrupt' event to the probe()
thread using the following method:
- probe(): write 0 to flag, enable s/w interrupt
- probe(): poll on flag, relax using usleep_range()
- ISR    : write 1 to flag

Replace with wake_up() / wait_event_timeout(). Besides being easier
to get right, this abstraction has better timing and memory
consistency properties.

Tested-by: Sven Van Asbroeck <thesven73@gmail.com> # lan7430
Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>
Link: https://lore.kernel.org/r/20201123191529.14908-2-TheSven73@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>