pci: split the function providing uio device and mappings

pci: split the function providing uio device and mappings

Add a new function pci_get_uio_dev() that parses /sys/bus/pci/devices
to get the uio device associated with a PCI device. This patch just
moves some code that was in pci_uio_map_resource() in the new function
without any functional change.

Thanks to this change, the next commit will be easier to understand.
Moreover it improves readability: having smaller functions help to
understand what pci_uio_map_resource() does.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>