osc: use consistent variable names for maxY and maxX

Signed-off-by: Tim Harder <timothy.harder@analog.com>
osc: use lower initial maxY values for markers

DAQ2 tests were running into issues since peak markers were below -100
in the case of no input so all the markers would end up on DC even
though a range including the DC peak was being ignored.

Signed-off-by: Tim Harder <timothy.harder@analog.com>