[rilmodem] Logging improvements to gprs

Rilmodem gprs logic slightly touched to store latest reg status
always to gprs_data struct. Reg status logged only when it changes
instead of always.
Merge pull request #256 from tkenakka/log-gprs

[rilmodem] Logging improvements to gprs
[RILMODEM] free before calling g_strdup to avoid mem leak

Signed-off-by: Jarko Poutiainen <Jarko.Poutiainen@oss.tieto.com>