drivers: can: Fix building with socket_can_generic.h

socket_can_generic.h makes some assumptions that are not always valid
for various drivers with regards to numbering.  To clean this up we
add defines for SOCKET_CAN_NAME_0 and SOCKET_CAN_NAME_2 in addition to
SOCKET_CAN_NAME_1.

We also move struct socket_can_context into the drivers themselves
since they know best how many CAN interfaces are getting supported and
what naming/number convention they'd have.

Additionally, this also exposes a few other build issues with the
can_mcux_flexcan driver that get fixed.

Finally, we remove the platform_allow from samples/net/sockets/can
since it is no longer needed.

Fixes #31733

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>