greybus: spi: rename rdwr field to xfer_flags

As more bits will be added to the field, let's make the field more
generic and name it accordingly. So, rename it from rdwr to xfer_flags.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>