spi: spi-mxs: Register the irq with the device name

Instead of registering the irq name with the driver name, it's better to pass
the device name so that we have a more explicit indication as to what spi
instance the irq is related:

$ cat /proc/interrupts
           CPU0
...
 27:          0         -  98  80014000.ssp

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