net/mlx5: fix premature disabling of interrupt

net/mlx5: fix premature disabling of interrupt

RXQ interrupts under Linux are based on the epoll mechanism. An expected
order of operations is as follows:
1. Call rte_eth_dev_rx_intr_enable(), to arm the CQ for receiving events
   on data input.
2. Block on rte_epoll_wait() with an array of file descriptors
   representing the CQ events. Upon data arrival the kernel will signal
   an input event on the corresponding CQ fd.
3. Call rte_eth_dev_rx_intr_disable() after the event was received and
   continue in polling mode. The mlx5 implementation of
   rte_eth_dev_rx_intr_disable() is to get the CQ event and ack it.

In practice applications may wake up from rte_epoll_wait() due to
timeout with no event to ack but still call
rte_eth_dev_rx_intr_disable() unconditionally.  In such cases the call
should return EAGAIN (since the file descriptors are non-blocked), as
opposed to EINVAL which indicates a real failure.  In case of EAGAIN the
PMD should not warn on "Unable to disable interrupt on Rx queue".

This commit fixes a earlier commit where the returned value 0 from
function devx_get_event() - was considered an error.

Fixes: 08d1838f645a ("net/mlx5: implement CQ for Rx using DevX API")

Signed-off-by: Ophir Munk <ophirmu@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Raslan Darawsheh <rasland@mellanox.com>