hw/tc58128.c: Remove unnecessary check for g_malloc failure

Remove a check for g_malloc failing: this never happens.
Also use g_malloc rather than g_malloc0 as we immediately
memset the entire region and so zero-initialising it is pointless.
Spotted by Coverity (see bug 887883).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
