otrx: make crc32 table global for further optimizations

So far we got only one generic function accessing this table, but
implementing optimizations will require calculating crc32 in other code
parts as well.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

Backport of r46077

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46092 3c298f89-4303-0410-b956-a3cf2f4a3e73