irqchip/gic-v3-its: Fix entry size mask for GITS_BASER

Entry Size in GITS_BASER<n> occupies 5 bits [52:48], but we mask out 8
bits.

Fixes: cc2d3216

 ("irqchip: GICv3: ITS command queue")
Cc: stable@vger.kernel.org
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>