media: dvb-frontends: Use kmemdup instead of duplicating its function

kmemdup has implemented the function that kmalloc() + memcpy().
We prefer to kmemdup rather than code opened implementation.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>