netfilter: nfnetlink_cthelper: Remove 'const' and '&' to avoid warnings

The related code can be simplified, and also can avoid related warnings
(with allmodconfig under parisc):

    CC [M]  net/netfilter/nfnetlink_cthelper.o
  net/netfilter/nfnetlink_cthelper.c: In function ‘nfnl_cthelper_from_nlattr’:
  net/netfilter/nfnetlink_cthelper.c:97:9: warning: passing argument 1 o ‘memcpy’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-array-qualifiers]
    memcpy(&help->data, nla_data(attr), help->helper->data_len);
           ^
  In file included from include/linux/string.h:17:0,
                   from include/uapi/linux/uuid.h:25,
                   from include/linux/uuid.h:23,
                   from include/linux/mod_devicetable.h:12,
                   from ./arch/parisc/include/asm/hardware.h:4,
                   from ./arch/parisc/include/asm/processor.h:15,
                   from ./arch/parisc/include/asm/spinlock.h:6,
                   from ./arch/parisc/include/asm/atomic.h:21,
                   from include/linux/atomic.h:4,
                   from ./arch/parisc/include/asm/bitops.h:12,
                   from include/linux/bitops.h:36,
                   from include/linux/kernel.h:10,
                   from include/linux/list.h:8,
                   from include/linux/module.h:9,
                   from net/netfilter/nfnetlink_cthelper.c:11:
  ./arch/parisc/include/asm/string.h:8:8: note: expected ‘void *’ but argument is of type ‘const char (*)[]’
   void * memcpy(void * dest,const void *src,size_t count);
          ^

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@soleta.eu>