greybus: introduce gb_operation_errno_map()

Define gb_operation_errno_map(), which maps an operation->errno
into the u8 value that represents it in the status field of an
operation response header.  It'll be used in an upcoming patch.

Make gb_operation_status_map() a private function.  It's not used
outside "operation.c" and I don't believe it ever should be.

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