slimbus: Fix out-of-bounds access in slim_slicesize()

With gcc-4.1.2:

    slimbus/messaging.c: In function ‘slim_slicesize’:
    slimbus/messaging.c:186: warning: statement with no effect

Indeed, clamp() is a macro not operating in-place, but returning the
clamped value.  Hence the value is not clamped at all, which may lead to
an out-of-bounds access.

Fix this by assigning the clamped value.

Fixes: afbdcc7c ("slimbus: Add messaging APIs to slimbus framework")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>