iio: st_magn: always define ST_MAGN_TRIGGER_SET_STATE

When CONFIG_IIO_TRIGGER is enabled but CONFIG_IIO_BUFFER is
not, we get a build error in the st_magn driver:

drivers/iio/magnetometer/st_magn_core.c:573:23: error: 'ST_MAGN_TRIGGER_SET_STATE' undeclared here (not in a function)
  .set_trigger_state = ST_MAGN_TRIGGER_SET_STATE,
                       ^~~~~~~~~~~~~~~~~~~~~~~~~

Apparently, this ST_MAGN_TRIGGER_SET_STATE macro was meant to
be set to NULL when the definition is not available because
st_magn_buffer.c is not compiled, but the alternative definition
was not included in the original patch. This adds it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 74f5683f ("iio: st_magn: Add irq trigger handling")
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>