PCI: epf-test: Cleanup pci_epf_test_cmd_handler()

PCI: epf-test: Cleanup pci_epf_test_cmd_handler()

Command codes are never combined together as flags into a single value.
Thus we can replace the series of "if" tests in pci_epf_test_cmd_handler()
with a cleaner switch-case statement.  This also allows checking that we
got a valid command and print an error message if we did not.

Link: https://lore.kernel.org/r/20230415023542.77601-11-dlemoal@kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>