drivers: nxp_enet: Remove unnecessary comments

Some comments in this driver are just redundant - the code
spells out exactly what the comment says. And some comment
blocks are just unnecessary to have. Finally, remove the TODO
comment because this is flagged by static analysis and there
are already tracking issues for the zero copy enhancement.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>