platform/mellanox: mlxreg-hotplug: Remove unused wait.h include

The driver does not make use of anything defined in wait.h.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
[dvhart: refactor into smaller functional changes, leave spinlock.h]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>