IB/iser: Change some module parameters to be RO

While we're at it, use permission defines instead
of octal values and rearrange a little bit.

Signed-off-by: Jenny Derzhavetz <jennyf@mellanox.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>