[posix-app] fix set radio spinel speed (#3157)

src/posix/platform/radio_spinel.cpp

@@ -459,7 +459,7 @@ static int OpenFile(const char *aFile, const char *aConfig)
             break;
         }
 
-        VerifyOrExit((rval = cfsetispeed(&tios, speed)) == 0, perror("cfsetispeed"));
+        VerifyOrExit((rval = cfsetspeed(&tios, speed)) == 0, perror("cfsetspeed"));
         VerifyOrExit((rval = tcsetattr(fd, TCSANOW, &tios)) == 0, perror("tcsetattr"));
         VerifyOrExit((rval = tcflush(fd, TCIOFLUSH)) == 0);
     }