drivers: can: rework can_attach_msgq

can_attach_msgq can be implemented as a wrapper of can_attach_isr.
This is implemented as a common function for all drives and reduces
the complexity of the specific drivers. Since this is common to
multi instances of drivers too, it is removed from the API struct.

Signed-off-by: Alexander Wachter <alexander.wachter@student.tugraz.at>