counter: 104-quad-8: Replace mutex with spinlock

This patch replaces the mutex I/O lock with a spinlock. This is in
preparation for a subsequent patch adding IRQ support for 104-QUAD-8
devices; we can't sleep in an interrupt context, so we'll need to use a
spinlock instead.

Acked-by: Syed Nayyar Waris <syednwaris@gmail.com>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Link: https://lore.kernel.org/r/3f74491dec66de10d062978bcb7b9c2b3bdea86c.1632884256.git.vilhelm.gray@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>