hw/riscv/riscv-iommu-sys.c: fix duplicated 'table_size'

hw/riscv/riscv-iommu-sys.c: fix duplicated 'table_size'

Trivial fix for the following ticket:

CID 1568580:  Incorrect expression  (EVALUATION_ORDER)
In "table_size = table_size = n_vectors * 16U",
    "table_size" is written twice with the same value.

Cc: qemu-trivial@nongnu.org
Cc: Peter Maydell <peter.maydell@linaro.org>
Resolves: Coverity CID 1568580
Fixes: 01c1caa9d1 ("hw/riscv/virt.c, riscv-iommu-sys.c: add MSIx support")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>