nvme: tweak unit test PCI config space accessors
nvme: tweak unit test PCI config space accessors

Make nvme_pcicfg_read32 set the referenced variable and make
nvme_pcicfg_write32 appear to read it so that the compiler doesn't warn
about unused/uninitialized data.

Change-Id: I4f06c0cca2fc11a8c6c5a60543c1c50a2f6a412d
Signed-off-by: Daniel Verkamp <7da385bb5c986b932d183f3a13d271ee45d259b4@intel.com>

changhuilin/spdk,changhuilin/spdk,changhuilin/spdk