greybus: define the invalid operation type symbolically

Use a symbolic constant (rather than just "0") to represent an
explicitly invalid operation type.  The protocols have all reserved
that value for that purpose--this just makes it explicit in the core
code (since we now leverage its existence).  Fix the code so it uses
the new symbolic value.

Define it in "operation.h" for all to see.  Move the common
definition of the GB_OPERATION_TYPE_RESPONSE flag mask there
as well.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>