greybus: spi: use timeout request send operation

When transfer speed is too slow (less than 17Khz) the operation can take
longer than the default greybus timeout. Because of this we need to use
the request_send_sync_timeout and calculate the correct timeout for each
operation.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Tested-by: Philip Yang <philipy@bsquare.com>
Tested-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>