via-cuda: Fix re-initialization of reply_ptr and reading_reply

via-cuda: Fix re-initialization of reply_ptr and reading_reply

When reading_reply is set, reply_ptr points into an adb_request struct.
Conversely, when reply_ptr instead points into the global cuda_rbuf,
reading_reply must be false.

Unfortunately, this rule can be violated because re-initialization
of reply_ptr and reading_reply presently depends on the TREQ input.

Fix this by re-initializing reply_ptr and reading_reply as soon as they
are known to be invalid.

Tested-by: Stan Johnson <[email protected]>
Signed-off-by: Finn Thain <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>