mlx4_core: Fix memory leak in mlx4_enable_msi_x()

When the msi_x option is enabled but pci_enable_msix() fails (not
enough vectors are available etc), the entries array was not freed on
the error path.

Signed-off-by: Nicolas Morey-Chaisemartin <nicolas.morey-chaisemartin@ext.bull.net>
Signed-off-by: Roland Dreier <rolandd@cisco.com>