IB/mlx4: Fix EQ deallocation in legacy mode

Commit e605b743 ("IB/mlx4: Increase the number of vectors (EQs)
available for ULPs") didn't handle correctly the case where there
aren't enough MSI-X vectors to increase the number of EQs, so only the
legacy EQs are allocated.  This results in an attempt to memset() to
zero the EQ table which was never allocated and a kernel crash.

Fix this by checking in the teardown flow if the table of EQs was ever
allocated.  Also remove some unneeded setting to zero of the EQ
related fields in struct mlx4_ib_dev.

Signed-off-by: Shlomo Pongratz <shlomop@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>