powerpc/powernv/sriov: fix unsigned int win compared to less than zero

Fix coccicheck warning:

  arch/powerpc/platforms/powernv/pci-sriov.c:443:7-10:
  WARNING: Unsigned expression compared with zero: win < 0

  arch/powerpc/platforms/powernv/pci-sriov.c:462:7-10:
  WARNING: Unsigned expression compared with zero: win < 0

Fixes: 39efc03e ("powerpc/powernv/sriov: Move M64 BAR allocation into a helper")
Reported-by: Tosk Robot <tencent_os_robot@tencent.com>
Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1605007170-22171-1-git-send-email-kaixuxia@tencent.com

arch/powerpc/platforms/powernv/pci-sriov.c

@@ -422,7 +422,7 @@ static int pnv_pci_vf_assign_m64(struct pci_dev *pdev, u16 num_vfs)
 {
 	struct pnv_iov_data   *iov;
 	struct pnv_phb        *phb;
-	unsigned int           win;
+	int                    win;
 	struct resource       *res;
 	int                    i, j;
 	int64_t                rc;