staging: octeon: Fix incorrect type in assignment

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.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>