iio: adc: ad7768-1: Fix ERR_PTR dereference in ad7768_fill_scale_tbl

iio: adc: ad7768-1: Fix ERR_PTR dereference in ad7768_fill_scale_tbl

The function iio_get_current_scan_type() can return an error pointer,
the return value scan_type is not checked for this and immediately
dereferenced which can cause a kernel panic.

Add check for IS_ERR() and propagate the error back.

Fixes: ff085189cb17 ("iio: adc: ad7768-1: add support for ADAQ776x-1 ADC Family")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/r/202602051234.5gArzLyZ-lkp@intel.com/
Signed-off-by: Ethan Tidmore <ethantidmore06@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>