dma-mapping: fix m32r build warning

kbuild test robot reports:

   In file included from include/linux/skbuff.h:34:0,
                    from include/linux/icmpv6.h:4,
                    from include/linux/ipv6.h:75,
                    from include/net/ipv6.h:16,
                    from include/linux/sunrpc/clnt.h:27,
                    from include/linux/nfs_fs.h:30,
                    from fs/lockd/clntlock.c:13:
   include/linux/dma-mapping.h: In function 'dma_map_resource':
>> include/linux/dma-mapping.h:274:16: warning: unused variable 'pfn' [-Wunused-variable]
     unsigned long pfn = __phys_to_pfn(phys_addr);
                   ^~~

The pfn value is only used once in the call to pfn_valid(), remove the
variable and calculate the pfn when it's needed. Note that the kbuild
report is old and PHYS_PFN() is now used instead of __phys_to_pfn() to
calculate the pfn.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

include/linux/dma-mapping.h

@@ -271,13 +271,12 @@ static inline dma_addr_t dma_map_resource(struct device *dev,
 					  unsigned long attrs)
 {
 	struct dma_map_ops *ops = get_dma_ops(dev);
-	unsigned long pfn = PHYS_PFN(phys_addr);
 	dma_addr_t addr;
 
 	BUG_ON(!valid_dma_direction(dir));
 
 	/* Don't allow RAM to be mapped */
-	BUG_ON(pfn_valid(pfn));
+	BUG_ON(pfn_valid(PHYS_PFN(phys_addr)));
 
 	addr = phys_addr;
 	if (ops->map_resource)