x86: fix return for arch_pcie_msi_vectors_allocate()

arch_pcie_msi_vectors_allocate() has a return type of uint8_t.
One of the error path returns -1 which would result in 255
being returned. So fix that by returning 0 instead, as there is
no vector being allocated anyway.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>