net: fill in MODULE_DESCRIPTION()s for SMSC drivers

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add descriptions to the SMSC 91x/911x/9420 Ethernet drivers.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>