tools: PCI: Fix broken pcitest compilation

pcitest is currently broken due to the following compiler error
and related warning. Fix by changing the run_test() function
signature to return an integer result.

pcitest.c: In function run_test:
pcitest.c:143:9: warning: return with a value, in function
returning void
  return (ret < 0) ? ret : 1 - ret; /* return 0 if test succeeded */

pcitest.c: In function main:
pcitest.c:232:9: error: void value not ignored as it ought to be
  return run_test(test);

Fixes: fef31eca ("tools: PCI: Fix compilation warnings")
Signed-off-by: Alan Mikhak <alan.mikhak@sifive.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>