spi: armada-3700: fix unsigned compare than zero on irq

spi->irq is an unsigned integer hence the check if status is less than
zero has no effect.  Fix this by replacing spi->irq with an int irq
so the less than zero compare will correctly detect errors.

Issue found with static analysis with CoverityScan, CID1388567

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Romain Perier <romain.perier@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>