bad switch, segfault in hw/pci-host/bonito.c bonito_readl

http://git.qemu.org/?p=qemu.git;a=blob;f=hw/pci-host/bonito.c;h=56292adb03cd1a9873c2c9e5a0b2978fd0572214;hb=master#l301

The switch statement is error-prone, since two branches return the same result.

Segfault reproducing steps:
1. make a Linux kernel(for example 3.16.0-rc2) with fuloong2e_defconfig
2. use 'qemu-system-mips64el -machine fulong2e' to boot the vmlinux

qemu versions tried: 2.0.0, 1.6.2