i40e: Fix memory leak at failure path in i40e_dbg_command_write()

The patch fixes a leak of 'cmd_buf' when copy_from_user() failed
in i40e_dbg_command_write().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>