IB/ipoib: Clean error paths in add port

Refactor error paths in ipoib_add_port() function. The code flow
ensures that the function terminates on every error flow and it makes
redundant all "else" cases.

The functions are called during the flow are returning "result < 0", in
case of error, so there is no need to check it explicitly.

Fixes: 58e9cc90 ("IB/IPoIB: Fix bad error flow in ipoib_add_port()")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>