kirkwood_spi: correct access to irq_mask register

kirkwood_spi: correct access to irq_mask register

Problem appears to have been present since day one but masked because alignment
aborts were not enabled. ca4b55800ed7 "arm, arm926ejs: always do cpu critical
inits" turned on alignment aborts and uncovered this latent problem.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Acked-By: Jason Cooper <u-boot@lakedaemon.net>
Tested-By: Holger Brunck <holger.brunck@keymile.com>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>