flexcan: disable bus error interrupts for the i.MX28

Due to a bug in most Flexcan cores, the bus error interrupt needs
to be enabled. Otherwise we don't get any error warning or passive
interrupts. This is _not_ necessary for the i.MX28 and this patch
disables bus error interrupts if "berr-reporting" is not requested.
This avoids bus error flooding, which might harm, especially on
low-end systems.

To handle such quirks of the Flexcan cores, a hardware feature flag
has been introduced, also replacing the "hw_ver" variable. So far
nobody could tell what Flexcan core version is available on what
Freescale SOC, apart from the i.MX6Q and P1010, and which bugs or
features are present on the various "hw_rev".

CC: Hui Wang <jason77.wang@gmail.com>
CC: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: David S. Miller <davem@davemloft.net>