RDMA/rxe: Remove type 2A memory window capability

Currently the rdma_rxe driver claims to support both 2A and 2B type memory
windows. But the IBA requires

	010-37.2.31: If an HCA supports the Base Memory Management
	extensions, the HCA shall support either Type 2A or Type 2B MWs,
	but not both.

This commit removes the device capability bit for type 2A memory windows
and adds a clarifying comment to rxe_mw.c.

Link: https://lore.kernel.org/r/20220407184321.14207-1-rpearsonhpe@gmail.com


Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>