drivers: can: Refactor drivers to use shared init priority

Refactors all of the CAN drivers to use a shared driver class
initialization priority configuration, CONFIG_CAN_INIT_PRIORITY, to
allow configuring CAN drivers separately from other devices. This is
similar to other driver classes like I2C and SPI.

The default is set to CONFIG_KERNEL_INIT_PRIORITY_DEVICE to preserve the
existing default initialization priority for most drivers. The exception
is the mcp2515 driver which has a dependency on a SPI driver and must
therefore initialize later than the default device priority.

Signed-off-by: Maureen Helm <maureen.helm@intel.com>