irqchip/irq-imx-gpcv2: Silence "fall through" warning

The -Wimplicit-fallthrough option requires that the /* fall through */
comment is placed in the 'case' statement that falls through, rather
than in the following one. Case seems to matter as well.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>