media: dvb_frontend: fix wrong cast in compat_ioctl

FE_GET_PROPERTY has always failed as following situations:
  - Use compatible ioctl
  - The array of 'struct dtv_property' has 2 or more items

This patch fixes wrong cast to a pointer 'struct dtv_property' from a
pointer of 2nd or after item of 'struct compat_dtv_property' array.

Signed-off-by: Katsuhiro Suzuki <suzuki.katsuhiro@socionext.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>