ARM: OMAP4: board-4430sdp: don't initialize value that is never used

sdp4430_spi_board_info.irq was initialized to ETH_KS8851_IRQ and in
omap_4430sdp_init() overwritten with gpio_to_irq(ETH_KS8851_IRQ) before
sdp4430_spi_board_info was registered. This is a bit confusing, so
better don't initialize .irq and document that it is set later.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>