isup, mtp3: Some functions have char* arguments that could be const char*

[InterLinked1]

We should be using const char* for arguments whenever possible.

Otherwise, applications may need to cast arguments to char*.

As an example:

chan_dahdi.c: In function ‘process_dahdi’:
chan_dahdi.c:19938:100: error: passing argument 2 of ‘ss7_set_isup_timer’ discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
19938 |                                 } else if (!ss7_set_isup_timer(link->ss7.ss7, strstr(v->name, ".") + 1, atoi(v->value))) {

chan_dahdi.c:19950:100: error: passing argument 2 of ‘ss7_set_mtp3_timer’ discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
19950 |                                 } else if (!ss7_set_mtp3_timer(link->ss7.ss7, strstr(v->name, ".") + 1, atoi(v->value))) {
      |