C#: Trigger.cs: Modify get_rate() / set_rate() to use Attr.read/write
C#: Trigger.cs: Modify get_rate() / set_rate() to use Attr.read/write

This is made because the functions iio_trigger_get_rate /
iio_trigger_set_rate are no longer available.

Signed-off-by: Paul Cercueil <73d292422978d45c9eeef455bb1b24a0db453722@analog.com>

gburca/libiio,sensarliar/libiio,analogdevicesinc/libiio,gburca/libiio,sensarliar/libiio,analogdevicesinc/libiio,sensarliar/libiio,Sunderfield/libiio,analogdevicesinc/libiio,Sunderfield/libiio,gburca/libiio,analogdevicesinc/libiio,Sunderfield/libiio