mlx5: Fixes to allow command polling mode to exist alongside event mode.

A command is either polling or event driven and the mode cannot change
during execution of a command. Make sure the event handler only handle
commands which are not polled. This is done by checking the command mode
in the command handler before completing commands.

Submitted by: hselasky@
Approved by: hselasky (mentor)
MFC after: 1 week
Sponsored by: Mellanox Technologies