rk1000: fix create sysfs file permission error.

Wrong permission will case following warning:

	------------[ cut here ]------------
	WARNING: at drivers/base/core.c:576 device_create_file+0x74/0xa4()
	Attribute rkcontrl: write permission without 'store'
	Modules linked in:
	CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.10.0 #247
	Call trace:
	[<ffffffc000087dc0>] dump_backtrace+0x0/0x10c
	[<ffffffc000087edc>] show_stack+0x10/0x1c
	[<ffffffc00087c0e4>] dump_stack+0x1c/0x28
	[<ffffffc0000a1b08>] warn_slowpath_common+0x74/0x9c
	[<ffffffc0000a1b7c>] warn_slowpath_fmt+0x4c/0x58
	[<ffffffc00036cfec>] device_create_file+0x70/0xa4
	[<ffffffc000399fc4>] rk1000_probe+0x1d0/0x20c
	[<ffffffc00058b190>] i2c_device_probe+0x9c/0xd4
	[<ffffffc000370e70>] driver_probe_device+0xd0/0x210
	[<ffffffc00037105c>] __driver_attach+0x60/0x90
	[<ffffffc00036f394>] bus_for_each_dev+0x60/0x88
	[<ffffffc00037096c>] driver_attach+0x1c/0x28
	[<ffffffc000370540>] bus_add_driver+0xf4/0x210
	[<ffffffc000371698>] driver_register+0xbc/0x138
	[<ffffffc00058b054>] i2c_register_driver+0x48/0xac
	[<ffffffc000bf2934>] rk1000_init+0x14/0x20
	[<ffffffc000081554>] do_one_initcall+0x80/0x120
	[<ffffffc000bdb8f0>] kernel_init_freeable+0x148/0x1e8
	[<ffffffc000876a10>] kernel_init+0x10/0xcc
	---[ end trace 1cc1e45d37744cff ]---

Signed-off-by: Zheng Yang <zhengyang@rock-chips.com>

drivers/mfd/rk1000-core.c

@@ -135,7 +135,7 @@ static ssize_t rk1000_show(struct device *dev,
 	return 0;
 }
 
-static DEVICE_ATTR(rkcontrl, 0666, rk1000_show, NULL);
+static DEVICE_ATTR(rkcontrl, S_IRUGO, rk1000_show, NULL);
 
 
 static int __init bootloader_cvbs_setup(char *str)