libsigrok.h: Don't store subdriver in sr_dev_driver

libsigrok.h: Don't store subdriver in sr_dev_driver

Not all drivers use subdrivers. The only reason the subdriver field was
introduced was to accomodate the model of serial-dmm.

The sr_dev_driver struct is available to the frontend. Exposing the subdriver
field creates the problem of exposing knowledge of libsigrok's internal driver
layout, even though the drivers are designed to be a flat list to the frontend.

Store the subdriver in the dev_context struct of serial-dmm.

Signed-off-by: Alexandru Gagniuc <redacted>