can: c_can: fix error checking of priv->instance in probe()

This patch adds a type cast from 'unsigned int' to 'int'.

'priv->instance' may less than zero, so need a type cast, the related
warnings (allmodconfig, "EXTRA_CFLAGS=-W"):

  drivers/net/can/c_can/c_can_platform.c:198:3: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>