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