iio: adis: Support different burst sizes

Add burst_max_len to `adis_burst`. This is useful for devices which
support different burst modes with different sizes. The buffer to be
used in the spi transfer is allocated with this variable making sure
that has space for all burst modes. The spi transfer length should hold
the "real" burst length depending on the current burst mode configured
in the device.

Moreover, `extra_len` in `adis_burst` is made const and it should
contain the smallest extra length necessary for a burst transfer. In
`struct adis` was added a new `burst_extra_len` that should hold the
extra bytes needed depending on the device instance being used.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>