drivers/mcp2515: extended can filters not applied

[firas-hamdi]

Contribution description

When using the extended CAN frames, I noticed that the filters were not correctly applied and they were considered as standard CAN filters. The reason for this bug is that after the check for filter type (extended or standard), the extended filter's flag is reset and any other later check will consider the filter as standard.

Testing procedure

Apply an extended CAN filter (29 bits) on one mailbox. It will be possible to receive CAN frames on that mailbox only if they have an extended CAN ID matching the filter

[benpicco]

This needs a rebase now

[wosym]

Should static analysis errors be fixed before merging? Not sure what the habits in RIOT are?

There's three in tests/candev/main.c