can: xilinx_can: fix comparison of unsigned variable

The variable err was of the type u32. It was being compared with < 0, and being
an unsigned variable the comparison would have been always false.

Moreover, err was getting the return value from set_reset_mode() and
xcan_set_bittiming(), and both are returning int.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>