HID: hyperv: remove unused struct synthhid_msg

struct synthhid_msg was meant to be a generic representation of the
possible protocol messages sent through VMBus. In practice, only the
header is read and depending on the message type, a cast to the actual
type is done. Also, SYNTHHID_MAX_INPUT_REPORT_SIZE constant isn't used
which I suspect is a leftover from the refactoring made while this
driver was at the staging folder.

This patch removes struct synthhid_msg and refactor the code
accordingly.

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>