net: can: remove custom hex_to_bin()

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>