ArmPkg/SemihostFs: replace SetMem with ZeroMem

SetMem arguments 2+3 are in the wrong order, resulting in
the call having no effect because Length is zero.

Fix this by using ZeroMem instead.

Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=4205
Reported-by: Jeremy Boone <jeremy.boone@nccgroup.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Rebecca Cran <rebecca@bsdio.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>

ArmPkg/Filesystem/SemihostFs/Arm/SemihostFs.c

@@ -574,7 +574,7 @@ ExtendFile (
   }
 
   Remaining = Size;
-  SetMem (WriteBuffer, 0, sizeof (WriteBuffer));
+  ZeroMem (WriteBuffer, sizeof (WriteBuffer));
   while (Remaining > 0) {
     WriteNb   = MIN (Remaining, sizeof (WriteBuffer));
     WriteSize = WriteNb;