drivers: can: mcp2515: fix build break

Fix build break introduced in 03ab7303:

/zephyr/drivers/can/can_mcp2515.c:871:55: error: 'const struct
mcp2515_config' has no member named 'int_pin'; did you mean 'int_gpio'?
  871 |   LOG_ERR("Unable to configure GPIO pin %u", dev_cfg->int_pin);

/zephyr/drivers/can/can_mcp2515.c:942:14: error: 'GPIO_DT_SPEC_INST_GET'
undeclared here (not in a function)
  942 |  .int_gpio = GPIO_DT_SPEC_INST_GET(0),
      |              ^~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>