Introduce standard cleanup helper

Introduce standard cleanup helper

std_init() allocates a drv_context struct which needs to be freed by the
driver in its cleanup struct. But the vast majority of drivers does never
does this causing memory leaks.

Instead of addressing the issue by manually adding code to free the struct
to each driver introduce a new helper function std_cleanup() that takes
care of this. In addition to freeing the drv_context struct std_cleanup()
also invokes sr_dev_clear() which takes care of freeing all devices
attached to the driver.

Combining both operations in the same helper function allows to use
std_cleanup() as the cleanup callback for all existing drivers, which
reduces the amount of boiler-plate code quite a bit.

All drivers are updated to use the new helper function.

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