[SIMB-1830] easycwmp: correct firmware download size check

[SIMB-1830] easycwmp: correct firmware download size check

Improve the AWK call used to get the available space in /tmp to
ensure it will not be output as a floating point number under busybox
AWK.

Fixes: a792f797c94b3a085a25ffd86063dc024d7d1182
[SIMB-1830] easycwmp: download size check before firmware update