Use bn_free to free big numbers

At some places of the code, some big numbers, allocated through
bn_alloc_max(), were freed using free().
They are now free using bn_free()

Signed-off-by: Pascal Brand <pascal.brand@st.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org> (STM platform)