net: tsec: Fix and cleanup tsec_mcast_addr()

There are several implementation issues for tsec_mcast_addr()
addressed by this patch:
* unmanaged, not portable r/w access to registers; fixed with
setbits_be32()/ clrbits_be32()
* use of volatile pointers
* unnecessary forced cast to u8 for the ether_crc() result
* removed redundant parens
* corrected some comment slips

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Patch: 279000