crypto: ccp/sev-dev-tsm - bail out early when pdev->bus is NULL

In the Linux kernel, the following vulnerability has been resolved:

crypto: ccp/sev-dev-tsm - bail out early when pdev->bus is NULL

dsm_create() initially checks pdev->bus when computing segment_id:

u8 segment_id = pdev->bus ? pci_domain_nr(pdev->bus) : 0;

But the next two lines unconditionally dereference pdev->bus via
pcie_find_root_port() and especially pci_dev_id(pdev), which expands
to PCI_DEVID(dev->bus->number, dev->devfn). If pdev->bus is in fact
NULL, segment_id is initialised to 0 but the very next statement
crashes the kernel.

smatch flags this:

drivers/crypto/ccp/sev-dev-tsm.c:253 dsm_create() error: we
previously assumed 'pdev->bus' could be null (see line 251)

Make the NULL handling consistent: if pdev->bus is NULL the device
has no PCI context to work with and SEV TIO setup cannot proceed,
so return -ENODEV before any of the bus-dependent lookups. The
remaining initialisation now runs only on the path where pdev->bus
is known to be valid.

No change for callers where pdev->bus is non-NULL, which is the
only case where dsm_create() did meaningful work before this change.