net/mlx4: use a single interrupt handle

net/mlx4: use a single interrupt handle

The reason one interrupt handle is currently used for RMV/LSC events and
another one for Rx traffic is because these come from distinct file
descriptors.

This can be simplified however as Rx interrupt file descriptors are stored
elsewhere and are registered separately.

Modifying the interrupt handle type to RTE_INTR_HANDLE_UNKNOWN has never
been necessary as disabling interrupts is actually done by unregistering
the associated callback (RMV/LSC) or emptying the EFD array (Rx). Instead,
make clear that the base handle file descriptor is invalid by setting it to
-1 when disabled.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>