dslogic: Implement continuous mode

For low sampling speeds (up to 25MHz) DSLogic offers a streaming mode where
samples are sent directly to the USB interface, like a fx2lafw device.

For high sampling speeds (up to 400MHz) only buffer mode is supported.

This commit allows the user to set which mode should be used. The configuration
is done by using SR_CONF_CONTINUOUS.

Signed-off-by: Diego Asanza <redacted>