emul: spi: SPI_EMUL_INIT(n) hard-coded to instance #0

The `SPI_EMUL_INIT(n)` macro used to instantiate SPI bus emulators
builds an array of SPI devices attached to the bus, but always checks
instance #0 of the bus and not `n`. This makes multiple instances of an
emulated SPI bus unusable and popualtes them all with bus 0's devices.

Change the `0` to `n`, which matches how the I2C bus emulator does it.

Signed-off-by: Tristan Honscheid <honscheid@google.com>

drivers/spi/spi_emul.c

@@ -120,7 +120,7 @@ static struct spi_driver_api spi_emul_api = {
 
 #define SPI_EMUL_INIT(n) \
  static const struct emul_link_for_bus emuls_##n[] = { DT_FOREACH_CHILD( \
- DT_DRV_INST(0), EMUL_LINK_AND_COMMA) }; \
+ DT_DRV_INST(n), EMUL_LINK_AND_COMMA) }; \
  static struct emul_list_for_bus spi_emul_cfg_##n = { \
  .children = emuls_##n, \
  .num_children = ARRAY_SIZE(emuls_##n), \