i2c: dev: Force case user pointers in compat_i2cdev_ioctl()

Sparse has warned us about wrong address space for user pointers:

  i2c-dev.c:561:50: warning: incorrect type in initializer (different address spaces)
  i2c-dev.c:561:50:    expected unsigned char [usertype] *buf
  i2c-dev.c:561:50:    got void [noderef] __user *

Force cast the pointer to (__u8 *) that is used by I²C core code.

Note, this is an additional fix to the previously addressed similar issue
in the I2C_RDWR case in the same function.

Fixes: 3265a7e6

 ("i2c: dev: Add __user annotation")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>