iio: events: Make iio_push_event() IRQ context save

Currently it is not save to call iio_push_event() from hard IRQ context since
the IIO event code uses spin_lock()/spin_unlock() and it is not save to mix
calls to spin_lock()/spin_unlock() from different contexts on the same lock.
E.g. if the lock is being held in iio_event_chrdev_read() and an interrupts
kicks in and the interrupt handler calls iio_push_event() we end uo with a
deadlock.

This patch updates iio_push_event() to use spin_lock_irqsave()/
spin_unlock_irqstrestore(), since it can be called from both IRQ and non-IRQ
context. All other other users of the lock, which are always run in non-IRQ
context, are updated to spin_lock_irq()/spin_unlock_irq().

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>