greybus: operation: allow atomic operation allocations

Add gfp mask argument to gb_operation_create to allow operations to be
allocated in atomic context.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>