common/mlx5: fix user mode register access command

common/mlx5: fix user mode register access command

To detect the timestamp mode configured on the NIC the mlx5
PMD uses the firmware command ACCESS_REGISTER_USER. This
command is relatively new and might be not supported by
older firmware versions and was rejected, causing annoying
messages in kernel log.

This patch adds the attribute flag check whether firmware
supports the command and avoid the call if it does not.

Fixes: bb7ef9a96281 ("common/mlx5: add register access DevX routine")

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>