netfilter: nf_tables: Use 32 bit addressing register from nft_type_to_reg()

nft_type_to_reg() needs to return the register in the new 32 bit addressing,
otherwise we hit EINVAL when using mappings.

Fixes: 49499c3e

 ("netfilter: nf_tables: switch registers to 32 bit addressing")
Reported-by: Andreas Schultz <aschultz@tpip.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>