spi: spi-imx: Do not store the irq number in the private structure

The irq number is only used inside the probe function, so there is really no
need to store it in the private structure.

Use a local 'irq' variable to hold the the irq number instead.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>