Fixed compilation warnings

drivers/rilmodem/sms.c: In function 'ril_read_sms_on_sim_cb':
drivers/rilmodem/sms.c:401:2: warning: pointer targets in passing argument 1 of 'strlen' differ in signedness [-Wpointer-sign]
//usr/include/string.h:399:15: note: expected 'const char *' but argument is of type 'const unsigned char *'
drivers/rilmodem/sms.c:393:14: warning: unused variable 'i' [-Wunused-variable]
drivers/rilmodem/sms.c:393:6: warning: variable 'sms_len' set but not used [-Wunused-but-set-variable]
drivers/rilmodem/sms.c: In function 'ril_new_sms_on_sim':
drivers/rilmodem/sms.c:440:10: warning: passing argument 3 of 'cb_data_new2' makes pointer from integer without a cast [enabled by default]
drivers/rilmodem/rilutil.h:147:31: note: expected 'void *' but argument is of type 'int'