iio: imu: adis16400: rename adis16400_core.c -> adi16400.c

This change does a simple 1-to-1 rename of the adis16400_core.c file. Now
that the `adis16400_buffer.c` file was removed, everything can be moved
into a single driver file.
But first, this rename.
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/imu/Makefile

@@ -4,7 +4,6 @@
 #
 
 # When adding new entries keep the list in alphabetical order
-adis16400-y             := adis16400_core.o
 obj-$(CONFIG_ADIS16400) += adis16400.o
 obj-$(CONFIG_ADIS16480) += adis16480.o