selftests/powerpc/papr_vpd: Check devfd before get_system_loc_code()

Calling get_system_loc_code before checking devfd and errno fails the test
when the device is not available, the expected behaviour is a SKIP.

Change the order of 'SKIP_IF_MSG' to correctly SKIP when the /dev/
papr-vpd device is not available.

Test output before:
  Test FAILED on line 271

Test output after:
  [SKIP] Test skipped on line 266: /dev/papr-vpd not present

Signed-off-by: R Nageswara Sastry <rnsastry@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240131130859.14968-1-rnsastry@linux.ibm.com