Staging:vt6656: Replace some kzalloc calls with kcalloc in iwctl.c

This replacement is justified by the fact that kcalloc could prevent integer
overflow resulted from the multiplications used in kzalloc.

The semantic patch that showed this suggestion is kzalloc.cocci

Signed-off-by: Iulia Manda <iulia.manda21@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>