rpi_power_switch: fix permissions to build on 3.18

  CC [M]  /home/pi/rpi-firmware/fbtft/workdir/fbtft_tools/rpi_power_switch/rpi_power_switch.o
In file included from include/linux/thread_info.h:11:0,
                 from include/asm-generic/preempt.h:4,
                 from arch/arm/include/generated/asm/preempt.h:1,
                 from include/linux/preempt.h:18,
                 from include/linux/spinlock.h:50,
                 from include/linux/seqlock.h:35,
                 from include/linux/time.h:5,
                 from include/linux/stat.h:18,
                 from include/linux/module.h:10,
                 from /home/pi/rpi-firmware/fbtft/workdir/fbtft_tools/rpi_power_switch/rpi_power_switch.c:12:
include/linux/bug.h:33:45: error: negative width in bit-field â<anonymous>â
 #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
                                             ^
include/linux/kernel.h:815:3: note: in expansion of macro âBUILD_BUG_ON_ZEROâ
   BUILD_BUG_ON_ZERO((perms) & 2) +    \
   ^
include/linux/sysfs.h:75:12: note: in expansion of macro âVERIFY_OCTAL_PERMISSIONSâ
    .mode = VERIFY_OCTAL_PERMISSIONS(_mode) },  \
            ^
include/linux/device.h:540:45: note: in expansion of macro â__ATTRâ
  struct device_attribute dev_attr_##_name = __ATTR(_name, _mode, _show, _store)
                                             ^
/home/pi/rpi-firmware/fbtft/workdir/fbtft_tools/rpi_power_switch/rpi_power_switch.c:282:8: note: in expansion of macro âDEVICE_ATTRâ
 static DEVICE_ATTR(do_shutdown, 0666, do_shutdown_show, do_shutdown_store);
        ^