s390/vx: remove comments from macros which break LLVM's IAS

LLVM's integrated assembler does not like comments within macros:

<instantiation>:3:19: error: too many positional arguments
        GR_NUM  b2, 1       /* Base register */
                            ^
Remove them, since they are obvious anyway.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>