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>