mfd: cros ec: spi: Increase EC transaction delay

50 us is not a long enough delay between EC transactions. At least 70 us
are needed for the 16 MHz STM32L part. Increase the delay to 200 us for
an extra safety margin.

Reviewed-by: Randall Spangler <rspangler@chromium.org>
Signed-off-by: Derek Basehore <dbasehore@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>