RDMA/rxe: Fix types in rxe_icrc.c

RDMA/rxe: Fix types in rxe_icrc.c

Currently the ICRC is generated as a u32 type and then forced to a __be32
and stored into the ICRC field in the packet. The actual type of the ICRC
is __be32. This patch replaces u32 by __be32 and eliminates the casts.
The computation is exactly the same as the original but the types are more
consistent.

Link: https://lore.kernel.org/r/20210707040040.15434-10-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>