net: move ether_crc to tsec driver

ether_crc was added to the core net code in commit 53a5c424
("multicast tftp: RFC2090") so that other drivers could use it. However
the only current user of it is tsec.c so move it there.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>