platform/chrome: cros_ec_spi: Wait for USECS, not NSECS

The use of `delay_usecs` in terminate_request() was replaced with the new
`delay` struct used by the SPI subsystem, however the unit was
set to SPI_DELAY_UNIT_NSECS instead of SPI_DELAY_UNIT_USECS. This fixes that.

Fixes: 7d3ca507

 ("platform/chrome: cros_ec_spi: Use new structure for SPI transfer delays")
Signed-off-by: Benson Leung <bleung@chromium.org>