Drop unnecessary struct sr_dev_driver forward declarations

Most drivers have a forward declaration to their sr_dev_driver struct at
the beginning of the driver file. This is due to historic reasons and often
no longer required. So remove all the unnecessary forward declarations.

Some drivers still require the forward declaration, but only reference the
driver struct from within the driver scan() callback. Since the driver
struct is passed to the scan callback replace the references to the global
variable with the local parameter. In some cases this requires adding the
parameter to some of the helper functions that are called from the scan()
callback.

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