PCI: endpoint: pci-epf-vntb: reduce several globals to statics

sparse reports
drivers/pci/endpoint/functions/pci-epf-vntb.c:975:5: warning: symbol 'pci_read' was not declared. Should it be static?
drivers/pci/endpoint/functions/pci-epf-vntb.c:984:5: warning: symbol 'pci_write' was not declared. Should it be static?
drivers/pci/endpoint/functions/pci-epf-vntb.c:989:16: warning: symbol 'vpci_ops' was not declared. Should it be static?

These functions and variables are only used in pci-epf-vntb.c, so their storage
class specifiers should be static.

Fixes: ff32fac00d97 ("NTB: EPF: support NTB transfer between PCI RC and EP connection")
Signed-off-by: Tom Rix <trix@redhat.com>
Acked-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>