hw/ppc/ppc440_uc: Remove dead code in sdram_size()

Coverity points out in CID 1390588 that the test for sh == 0
in sdram_size() can never fire, because we calculate sh with
    sh = 1024 - ((bcr >> 6) & 0x3ff);
which must result in a value between 1 and 1024 inclusive.

Without the relevant manual for the SoC, we're not completely
sure of the correct behaviour here, but we can remove the
dead code without changing how QEMU currently behaves.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>