dell-rbtn: Ignore ACPI notifications if device is suspended

Some BIOSes unconditionally send an ACPI notification to RBTN when the
system is resuming from suspend. This makes dell-rbtn send an input
event to userspace as if a function key was pressed. Prevent this by
ignoring all the notifications received while the device is suspended.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=106031
Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Tested-by: Alex Hung <alex.hung@canonical.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Darren Hart <dvhart@linux.intel.com>