RDMA/bnxt_re: Remove unsupported bnxt_re_modify_ah callback

There is no need to return always zero for function which is not
supported, especially since 0 is the wrong return code.

Link: https://lore.kernel.org/r/20211102073054.410838-1-kamalheib1@gmail.com


Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>