can: peak: fix potential bug in packet fragmentation

In some rare conditions when running one PEAK USB-FD interface over
a non high-speed USB controller, one useless USB fragment might be sent.
This patch fixes the way a USB command is fragmented when its length is
greater than 64 bytes and when the underlying USB controller is not a
high-speed one.

Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>