net/mlx5: Fix incorrect raw command length parsing

The NULL character was not set correctly for the string containing
the command length, this caused failures reading the output of the
command due to a random length. The fix is to initialize the output
length string.

Fixes: e126ba97

 ("mlx5: Add driver for Mellanox Connect-IB adapters")
Signed-off-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>