Revert "syscall: fix dereference of undefined pointer"

This reverts commit bc658e4a.

Some versions of gcc warn about this:

linux-user/syscall.c: In function ‘do_ioctl_rt’:
linux-user/syscall.c:5577:37: error: ‘host_rt_dev_ptr’ may be used uninitialized in this function [-Werror=uninitialized]

and in particular the Travis builds fail; they use
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3.

Revert the change to fix the travis builds.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>