iio: adis: add burst_max_speed_hz variable

iio: adis: add burst_max_speed_hz variable

Typically, in burst mode, the device cannot operate at it's full spi
speed. Hence, the spi transfers for burst mode have to take this into
account. With this change we avoid a potential race with the spi core as
drivers were 'hacking' the device 'max_speed_hz' directly in the
trigger handler.

Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210427085454.30616-5-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>