nes_unregister_ofa_device() dereferences the nesibdev pointer before
testing if it's NULL.  Also, the test is doubly redundant because the
only caller of nes_unregister_ofa_device() is nes_destroy_ofa_device(),
which already tests if nesibdev is NULL.  Remove the unnecessary test.

This was spotted by the Coverity checker (CID 2190).
Signed-off-by: Roland Dreier <rolandd@cisco.com>