net: marvell: prestera: Use dscp_t in struct prestera_kern_fib_cache

Use the new dscp_t type to replace the kern_tos field of struct
prestera_kern_fib_cache. This ensures ECN bits are ignored and makes it
compatible with the dscp fields of struct fib_entry_notifier_info and
struct fib_rt_info.

This also allows sparse to flag potential incorrect uses of DSCP and
ECN bits.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Yevhen Orlov <yevhen.orlov@plvision.eu>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>