chan_sip.c: Validation on module reload

Change validation on reload module because now used the cli function for
reload. The sip_reload() function never fail and ever return NULL for this
reason on reload() now use the call the sip_reload() and return
AST_MODULE_LOAD_SUCCESS.

This problem is dectected on reload by PUT method on ARI, getting always
404 http code when the module is reloaded.

ASTERISK-25325 #close
Reporte by: Rodrigo Ramírez Norambuena

Change-Id: I41215877fb2cfc589e0d4d464000cf6825f4d7fb