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>

drivers/net/ethernet/silan/sc92031.c

@@ -251,7 +251,6 @@ enum PMConfigBits {
  * use of mdelay() at _sc92031_reset.
  * Functions prefixed with _sc92031_ must be called with the lock held;
  * functions prefixed with sc92031_ must be called without the lock held.
- * Use mmiowb() before unlocking if the hardware was written to.
  */
 
 /* Locking rules for the interrupt: