[MDC-825] simetbox-openwrt-simet-ma: fix simet_ma rpc geolocate method

[MDC-825] simetbox-openwrt-simet-ma: fix simet_ma rpc geolocate method

Return geolocation output as JSON strings, due to OpenWRT Chaos Calmer
issues.  Negative floats and u64 output work well on 18.06 and later,
but 15.05 can easily trip on them and return an ubus error, instead.

Correctly fetch the input parameters to the geolocate method.

Fixes: commit cd73b4b54c43869d5301917eede81516cc57c05a
"[MDC-825] simetbox-openwrt-simet-ma: add simet_ma rpcd service"