iio: accel: bmc150: Fix irq assumption regression

The code in bmc150-accel-core.c unconditionally calls
bmc150_accel_set_interrupt() in the iio_buffer_setup_ops,
such as on the runtime PM resume path giving a kernel
splat like this if the device has no interrupts:

Unable to handle kernel NULL pointer dereference at virtual
address 00000001 when read

PC is at bmc150_accel_set_interrupt+0x98/0x194
LR is at __pm_runtime_resume+0x5c/0x64
(...)
Call trace:
bmc150_accel_set_interrupt from bmc150_accel_buffer_postenable+0x40/0x108
bmc150_accel_buffer_postenable from __iio_update_buffers+0xbe0/0xcbc
__iio_update_buffers from enable_store+0x84/0xc8
enable_store from kernfs_fop_write_iter+0x154/0x1b4

This bug seems to have been in the driver since the beginning,
but it only manifests recently, I do not know why.

Store the IRQ number in the state struct, as this is a common
pattern in other drivers, then use this to determine if we have
IRQ support or not.

Cc: [e-mail]
Signed-off-by: Linus Walleij <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Reviewed-by: Nuno Sá <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/accel/bmc150-accel-core.c

···
 	const struct bmc150_accel_interrupt_info *info = intr->info;
 	int ret;
 
+	/* We do not always have an IRQ */
+	if (data->irq <= 0)
+		return 0;
+
 	if (state) {
 		if (atomic_inc_return(&intr->users) > 1)
 			return 0;
···
 	}
 
 	if (irq > 0) {
+		data->irq = irq;
 		ret = devm_request_threaded_irq(dev, irq,
 						bmc150_accel_irq_handler,
 						bmc150_accel_irq_thread_handler,

drivers/iio/accel/bmc150-accel.h

···
 
 struct bmc150_accel_data {
 	struct regmap *regmap;
+	int irq;
 	struct regulator_bulk_data regulators[2];
 	struct bmc150_accel_interrupt interrupts[BMC150_ACCEL_INTERRUPTS];
 	struct bmc150_accel_trigger triggers[BMC150_ACCEL_TRIGGERS];