net: fill in MODULE_DESCRIPTION()s for PCS drivers

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add descriptions to the Lynx, XPCS and LynxI PCS drivers.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>