ARM: 7317/1: irq: avoid NULL check in for_each_irq_desc loop

ARM unconditionally selects CONFIG_GENERIC_HARDIRQS, so the definition
of for_each_irq_desc will check that the desc is non-NULL anyway.

This patch removes a redundant check from the IRQ migration code.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>