net: cs89x0: add MODULE_LICENSE

This driver lacks a MODULE_LICENSE tag, leading to a Kbuild warning:

WARNING: modpost: missing MODULE_LICENSE() in drivers/net/ethernet/cirrus/cs89x0.o

This adds license, author, and description according to the
comment block at the start of the file.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>