net: can: comparison of unsigned 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>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>