Check whether a sample limit was actually set (> 0) before checking if
that sample limit is reached. This also fixes continuous acquisition mode
for drivers which have that.
This is the driver model agreed upon for all drivers.
As a result of the split, a devc->num_probes field had to be added in order to
reduce the interdependence between api.c and protocol.c .
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>