can: Use correct type in sizeof() in nla_put()

The sizeof() is invoked on an incorrect variable, likely due to some
copy-paste error, and this might result in memory corruption. Fix this.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: netdev@vger.kernel.org
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>