Tx doorbell is handled by txdb_tasklet and doesn't
have an associated IRQ.

Anyhow, imx_mu_shutdown ignores this and tries to
free an IRQ that wasn't requested for Tx DB resulting
in the following warning:

[    1.967644] Trying to free already-free IRQ 26
[    1.972108] WARNING: CPU: 2 PID: 157 at kernel/irq/manage.c:1708 __free_irq+0xc0/0x358
[    1.980024] Modules linked in:
[    1.983088] CPU: 2 PID: 157 Comm: kworker/2:1 Tainted: G
[    1.993524] Hardware name: Freescale i.MX8QXP MEK (DT)
[    1.998668] Workqueue: events deferred_probe_work_func
[    2.003812] pstate: 60000085 (nZCv daIf -PAN -UAO)
[    2.008607] pc : __free_irq+0xc0/0x358
[    2.012364] lr : __free_irq+0xc0/0x358
[    2.016111] sp : ffff00001179b7e0
[    2.019422] x29: ffff00001179b7e0 x28: 0000000000000018
[    2.024736] x27: ffff000011233000 x26: 0000000000000004
[    2.030053] x25: 000000000000001a x24: ffff80083bec74d4
[    2.035369] x23: 0000000000000000 x22: ffff80083bec7588
[    2.040686] x21: ffff80083b1fe8d8 x20: ffff80083bec7400
[    2.046003] x19: 0000000000000000 x18: ffffffffffffffff
[    2.051320] x17: 0000000000000000 x16: 0000000000000000
[    2.056637] x15: ffff0000111296c8 x14: ffff00009179b517
[    2.061953] x13: ffff00001179b525 x12: ffff000011142000
[    2.067270] x11: ffff000011129f20 x10: ffff0000105da970
[    2.072587] x9 : 00000000ffffffd0 x8 : 0000000000000194
[    2.077903] x7 : 612065657266206f x6 : ffff0000111e7b09
[    2.083220] x5 : 0000000000000003 x4 : 0000000000000000
[    2.088537] x3 : 0000000000000000 x2 : 00000000ffffffff
[    2.093854] x1 : 28b70f0a2b60a500 x0 : 0000000000000000
[    2.099173] Call trace:
[    2.101618]  __free_irq+0xc0/0x358
[    2.105021]  free_irq+0x38/0x98
[    2.108170]  imx_mu_shutdown+0x90/0xb0
[    2.111921]  mbox_free_channel.part.2+0x24/0xb8
[    2.116453]  mbox_free_channel+0x18/0x28

This bug is present from the beginning of times.

Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>