staging: octeon: Fix incorrect type in assignment

Fix the following warning generated by sparse in
drivers/staging/octeon/ethernet-tx.c:

drivers/staging/octeon/ethernet-tx.c:563:50: warning: incorrect type in assignment (different base types)
drivers/staging/octeon/ethernet-tx.c:563:50:    expected unsigned short [usertype] hw_chksum
drivers/staging/octeon/ethernet-tx.c:563:50:    got restricted __wsum [usertype] csum

Warning generated by running:
make C=2 CF="-D__CHECK_ENDIAN__" drivers/staging/octeon/
Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Link: https://lore.kernel.org/r/20191010043815.14027-1-wambui.karugax@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/octeon/octeon-stubs.h

@@ -123,7 +123,7 @@ union cvmx_pip_wqe_word0 {
 	struct {
 		uint64_t next_ptr:40;
 		uint8_t unused;
-		uint16_t hw_chksum;
+		__wsum hw_chksum;
 	} cn38xx;
 	struct {
 		uint64_t pknd:6;        /* 0..5 */