osc: workaround IIO signed integer limitation

- in case frequency reads negative adjust the value

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
osc: increase sample count limit

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Merge branch 'master' of https://github.com/analogdevicesinc/iio-oscilloscope