staging: iio: accel: Rename source files and update Makefile

Rename adis162xx_core.c files to adis162xx.c because
these are the only source files for these drivers.

Update Makefile to correspond to the new file names.

Signed-off-by: Narcisa Ana Maria Vasile <narcisaanamaria12@gmail.com>

 rename drivers/staging/iio/accel/{adis16201_core.c => adis16201.c} (100%)
 rename drivers/staging/iio/accel/{adis16203_core.c => adis16203.c} (100%)
 rename drivers/staging/iio/accel/{adis16209_core.c => adis16209.c} (100%)
 rename drivers/staging/iio/accel/{adis16240_core.c => adis16240.c} (100%)
Signed-off-by: Jonathan Cameron <jic23@kernel.org>