mfd: cros-ec: Fix host command buffer size

For SPI, we can get up to 32 additional bytes for response preamble.
The current overhead (2 bytes) may cause problems when we try to receive
a big response. Update it to 32 bytes.

Without this fix we could see a kernel BUG when we receive a big response
from the Chrome EC when is connected via SPI.

Signed-off-by: Vic Yang <victoryang@google.com>
Tested-by: Enric Balletbo i Serra <enric.balletbo.collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>