greybus: fix uart request_operation()

This fixes a problem similar to what was found in the battery
protcool driver code.

There's no need to allocate a local buffer, that already set up
by gb_operation_create().  Just use that instead.

Change a few variable names to reflect that they hold response
messages, not request messages.

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