IB/hfi1: Add explicit cast OPA_MTU_8192 to 'enum ib_mtu'

IB/hfi1: Add explicit cast OPA_MTU_8192 to 'enum ib_mtu'

Clang warns:

drivers/infiniband/hw/hfi1/qp.c:198:9: warning: implicit conversion from enumeration type 'enum opa_mtu' to different enumeration type 'enum ib_mtu' [-Wenum-conversion]
                mtu = OPA_MTU_8192;
                    ~ ^~~~~~~~~~~~

enum opa_mtu extends enum ib_mtu. There are typically two ways to deal
with this:

* Remove the expected types and just use 'int' for all parameters and
  types.

* Explicitly cast the enums between each other.

This driver chooses to do the later so do the same thing here.

Fixes: 6d72344cf6c4 ("IB/ipoib: Increase ipoib Datagram mode MTU's upper limit")
Link: https://lore.kernel.org/r/20200623005224.492239-1-natechancellor@gmail.com
Link: https://github.com/ClangBuiltLinux/linux/issues/1062
Link: https://lore.kernel.org/linux-rdma/20200527040350.GA3118979@ubuntu-s3-xlarge-x86/
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>