x86: ia32_signal.c: remove unnecessary cast to u32

__put_user() looks type of the 2nd parameter, so casting the 1st parameter
is not necessary.

   text	   data	    bss	    dec	    hex	filename
   6227	      0	      8	   6235	   185b	ia32_signal.o.new
   6227	      0	      8	   6235	   185b	ia32_signal.o.old

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>