qca_spi: Improve SPI IRQ handling

The functions qcaspi_netdev_open/close are responsible of request &
free of the SPI interrupt, which wasn't the best choice because
allocation problems are discovered not during probe. So let us split
IRQ allocation & enabling, so we can take advantage of a device
managed IRQ.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>