Remove unnecessary driver context checks

Remove unnecessary driver context checks

Some drivers check in some of their driver callbacks if the driver has been
initialized and return an error if it has not.

For the scan() callback the sigrok core checks if the driver has been
initialized and if not returns an error. So it is not possible that the
scan() callback gets called if the driver is not initialized. Without the
scan() callback succeeding it is not possible to get a reference to a
device which is associated with the driver, so it is not possible that any
of the device specific callbacks is called without the driver first being
initialized either.

In conclusion these checks are not necessary since they never evaluate to
true and can be dropped. If they should ever become necessary they should
be done in the sigrok core so all drivers and all callbacks are equally
handled.

Signed-off-by: Lars-Peter Clausen <redacted>