iio: Fix iio_buffer_register stub signature

Match the iio_buffer_register stub signature up to the real function and make
the second parameter const. This fixes a the following warnings if
CONFIG_IIO_BUFFER is disabled:

	drivers/staging/iio/accel/adis16201_core.c: In function ‘adis16201_probe’:
	drivers/staging/iio/accel/adis16201_core.c:536: warning: passing argument 2 of ‘iio_buffer_register’ discards qualifiers from pointer target type
	drivers/staging/iio/accel/adis16203_core.c: In function ‘adis16203_probe’:
	drivers/staging/iio/accel/adis16203_core.c:468: warning: passing argument 2 of ‘iio_buffer_register’ discards qualifiers from pointer target type
	drivers/staging/iio/accel/adis16204_core.c: In function ‘adis16204_probe’:
	drivers/staging/iio/accel/adis16204_core.c:527: warning: passing argument 2 of ‘iio_buffer_register’ discards qualifiers from pointer target type
	drivers/staging/iio/accel/adis16209_core.c: In function ‘adis16209_probe’:
	drivers/staging/iio/accel/adis16209_core.c:542: warning: passing argument 2 of ‘iio_buffer_register’ discards qualifiers from pointer target type
	drivers/staging/iio/accel/adis16240_core.c: In function ‘adis16240_probe’:
	drivers/staging/iio/accel/adis16240_core.c:588: warning: passing argument 2 of ‘iio_buffer_register’ discards qualifiers from pointer target type

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>