net: thunderbolt: Fix sparse warnings in tbnet_xmit_csum_and_map()

Fixes the following warning when the driver is built with sparse checks
enabled:

main.c:993:23: warning: incorrect type in initializer (different base types)
main.c:993:23:    expected restricted __wsum [usertype] wsum
main.c:993:23:    got restricted __be32 [usertype]

No functional changes intended.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>