RDMA/core: Add the function ib_mtu_int_to_enum

As the functionality to convert the MTU from a number to enum_ib_mtu
is ubiquitous, define a dedicated function and remove the duplicated
code.

Signed-off-by: Ram Amrani <Ram.Amrani@cavium.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>