PCI/switchtec: Factor out Gen3 ioctl_flash_part_info()

Refactor ioctl_flash_part_info() into a Gen3-specific function because the
registers for flash partition information have changed significantly in
Gen4 and will require a completely different implementation.

No functional changes intended.

Co-developed-by: Kelvin Cao <kelvin.cao@microchip.com>
Link: https://lore.kernel.org/r/20200115035648.2578-4-logang@deltatee.com


Signed-off-by: Kelvin Cao <kelvin.cao@microchip.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>