Add an error check to cros_write_firmware if arguments are passed.
Add an error check to cros_write_firmware if arguments are passed.

This gives a better error message if someone is using the script
incorrectly.

BUG=none
TEST=Tried calling with no extra args, 1 extra arg, and 2 extra args.

Change-Id: I2dc635fc1ba5c114f1eb239445b322733c37e260
Reviewed-on: http://gerrit.chromium.org/gerrit/4116
Tested-by: Doug Anderson <49983c4a76b998672954ed5edf4a62db4a336502@chromium.org>
Reviewed-by: Anton Staaf <84531b4b964c017023781514a6665b71e84b32bc@chromium.org>

coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,mgireesh05/dev-util,coreos/dev-util,coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util