net/ethernet/silan/sc92031: Remove stale comment about mmiowb()

mmiowb() is no more. It has ceased to be. It is an ex-barrier. So remove
all references to it from comments.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>