iio: imu: adis: Fix NULL pointer dereference in adis_init

The adis_init() function dereferences adis->ops to check if the
individual function pointers (write, read, reset) are NULL, but does
not first check if adis->ops itself is NULL.

Drivers like adis16480, adis16490, adis16545 and others do not set
custom ops and rely on adis_init() assigning the defaults. Since struct
adis is zero-initialized by devm_iio_device_alloc(), adis->ops is NULL
when adis_init() is called, causing a NULL pointer dereference:

    Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
    pc : adis_init+0xc0/0x118
    Call trace:
     adis_init+0xc0/0x118
     adis16480_probe+0xe0/0x670

Fix this by checking if adis->ops is NULL before dereferencing it,
falling through to assign the default ops in that case.

Fixes: 3b29bcee ("iio: imu: adis: Add custom ops struct")
Signed-off-by: Radu Sabau <radu.sabau@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>