- check for CONDIF_SYS_DCACHE_OFF or CONFIG_SYS_ARM_CACHE_WRITETHROUGH

- check for CONDIF_SYS_DCACHE_OFF or CONFIG_SYS_ARM_CACHE_WRITETHROUGH
  to prevent driver from being used with WB cache and add an
  explanatory comment
- invalidate cache for packet buffers in fec_rbd_init()
- move cache flush/invalidate operations closer to the point where
  they are actually required
- use correct parameters for cache operations