uio: fix compilation with -Og

uio: fix compilation with -Og

The compilation with gcc-6.3.0 and EXTRA_CFLAGS=-Og gives the following
error:

  CC eal_pci_uio.o
  eal_pci_uio.c: In function ‘pci_get_uio_de ’:
  eal_pci_uio.c:221:9: error: ‘uio_num’ may be used uninitialized in
                       this function [-Werror=maybe-uninitialized]
  return uio_num;
         ^~~~~~~

This is a false positive: gcc is not able to see that when e != NULL,
uio_num is always set. Fix the warning by initializing uio_num to -1,
and by the way, change the type to int.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>