cros_ec: Handling EC_CMD_GET_NEXT_EVENT

cros_ec: Handling EC_CMD_GET_NEXT_EVENT

With commit 690079767803 ("cros_ec: Support keyboard scanning with
EC_CMD_GET_NEXT_EVENT") check_for_keys() tries to read keyboard
strokes using EC_CMD_GET_NEXT_EVENT. But the sandbox driver does
not understand this command. We need to reply with
-EC_RES_INVALID_COMMAND to force check_for_keys() to fall back to
use EC_CMD_MKBP_STATE. Currently the driver prints

    ** Unknown EC command 0x67

in this case. With the patch the message is suppressed.

In a future patch we should upgrade the sandbox driver to provide
EC_CMD_GET_NEXT_EVENT support.

Fixes: 690079767803 ("cros_ec: Support keyboard scanning with EC_CMD_GET_NEXT_EVENT")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>