mfd: cros_ec: Delay for 50ms when we see EC_CMD_REBOOT_EC

If someone sends a EC_CMD_REBOOT_EC to the EC, the EC will likely be
unresponsive for quite a while.  Add a delay to the end of the command
to prevent random failures of future commands.

NOTES:
* This could be optimized a bit by simply delaying the next command
  sent, but EC_CMD_REBOOT_EC is such a rare command that the extra
  complexity doesn't seem worth it.
* This is a bit of an "ugly hack" since the SPI driver is effectively
  snooping on the communication and making a lot of assumptions.  It
  would be nice to architect in some better solution long term.
* This same logic probably needs to be applied to the i2c driver.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Lee Jones <lee.jones@linaro.org>