PCI: Drop msix_table_offset_reg() and msix_pba_offset_reg() macros

msix_table_offset_reg() is used only once and adds a useless indirection,
so just use the table offset directly.

msix_pba_offset_reg() is unused, so just delete it.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>