media: imx-pxp: Initialize the spinlock prior to using it

After devm_request_threaded_irq() is called there is a chance that an
interrupt may occur before the spinlock is initialized, which will trigger
a kernel oops.

To prevent that, move the initialization of the spinlock prior to
requesting the interrupts.

Fixes: 51abcf7f ("media: imx-pxp: add i.MX Pixel Pipeline driver")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>