spi/mpc8xxx: don't check platform_get_irq's return value against zero

platform_get_irq returns -ENXIO on failure, so !irq was probably
always true.  Make irq a signed variable and compare irq <= 0.  Note
that a return value of zero is still handled as error even though this
could mean irq0.

This is a followup to 305b3228 that
changed the return value of platform_get_irq from 0 to -ENXIO on error.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>