S2io: Fixed the case when the card initialization fails on mtu change

Fix the case when the card initialization fails on a mtu change and then
close is called (due to ifdown), which frees non existent rx buffers.
- Returning appropriate error codes in init_nic function.
- In s2io_close function s2io_card_down is called only when device is up.
- In s2io_change_mtu function return value of s2io_card_up function
  is checked and returned if it failed.

Signed-off-by: Surjit Reang <surjit.reang@neterion.com>
Signed-off-by: Sreenivasa Honnur <sreenivasa.honnur@neterion.com>
Signed-off-by: Ramkrishna Vepa <ram.vepa@neterion.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>