platform/chrome: cros_ec_spi: Document missing function parameters

Kerneldoc expects all kernel function members to be documented.

Fixes the following W=1 level warnings:

  cros_ec_spi.c:153: warning: Function parameter or member 'ec_dev' not described in 'receive_n_bytes'
  cros_ec_spi.c:153: warning: Function parameter or member 'buf' not described in 'receive_n_bytes'
  cros_ec_spi.c:153: warning: Function parameter or member 'n' not described in 'receive_n_bytes'

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>