mailbox: imx: fix suspend failue

imx_mu_isr() always calls pm_system_wakeup() even when it should not,
making the system unable to enter sleep.

Suspend fails as follows:
 armadillo:~# echo mem > /sys/power/state
 [ 2614.602432] PM: suspend entry (deep)
 [ 2614.610640] Filesystems sync: 0.004 seconds
 [ 2614.618016] Freezing user space processes ... (elapsed 0.001 seconds) done.
 [ 2614.626555] OOM killer disabled.
 [ 2614.629792] Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
 [ 2614.638456] printk: Suspending console(s) (use no_console_suspend to debug)
 [ 2614.649504] PM: Some devices failed to suspend, or early wake event detected
 [ 2614.730103] PM: resume devices took 0.080 seconds
 [ 2614.741924] OOM killer enabled.
 [ 2614.745073] Restarting tasks ... done.
 [ 2614.754532] PM: suspend exit
 ash: write error: Resource busy
 armadillo:~#

Upstream commit 892cb524 is correct, so this seems to be a
mistake during cherry-pick.

Cc: <stable@vger.kernel.org>
Fixes: a16f5ae8 ("mailbox: imx: fix wakeup failure from freeze mode")
Signed-off-by: Daisuke Mizobuchi <mizo@atmark-techno.com>
Reviewed-by: Dominique Martinet <dominique.martinet@atmark-techno.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>