irqchip/loongson-pch-msi: Use bitmap_zalloc() to allocate bitmap

Currently we use bitmap_alloc() to allocate msi bitmap which should be
initialized with zero. This is obviously wrong but it works because msi
can fallback to legacy interrupt mode. So use bitmap_zalloc() instead.

Fixes: 632dcc2c ("irqchip: Add Loongson PCH MSI controller")
Cc: stable@vger.kernel.org
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210209071051.2078435-1-chenhuacai@loongson.cn