riscv/barrier: Add missing space after ','

riscv/barrier: Add missing space after ','

The past form of RISCV_FENCE would cause checkpatch.pl to issue
error messages, the example is as follows:
ERROR: space required after that ',' (ctx:VxV)
26: FILE: arch/riscv/include/asm/barrier.h:27:
+#define __smp_mb()         RISCV_FENCE(rw,rw)
                                          ^
fix the remaining of RISCV_FENCE.

Signed-off-by: Eric Chan <ericchancf@google.com>
Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
Tested-by: Samuel Holland <samuel.holland@sifive.com>
Link: https://lore.kernel.org/r/20240217131328.3669364-1-ericchancf@google.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>