[posix] deassert DTR and RTS on macOS too with uart-init-deassert (#13750)

`uart-init-deassert` deasserts DTR and RTS for radios whose reset and
bootloader pins are driven by those lines, except under
`#ifndef __APPLE__`: on macOS the parameter only cleared flow control
and the lines stayed asserted, silently. #10454 added the guard because
a macOS CI job failed on the unguarded version, which deasserted on
every open, and the author had no Mac to look further; #10644 then made
the deassert opt-in, so CI does not reach the ioctl today.

macOS has the ioctl: sys/ttycom.h declares TIOCMBIC, TIOCM_DTR and
TIOCM_RTS, and IOSerialBSDClient::iossioctl() passes the bits through
to the serial driver's state. Drop the guard so the parameter behaves
the same everywhere, and record the ioctl's result in rval so a tty
that rejects it fails the open as the other termios steps do, instead
of leaving the port half configured.

src/posix/platform/hdlc_interface.cpp

@@ -636,17 +636,9 @@ int HdlcInterface::OpenFile(const Url::Url &aRadioUrl)
         else if (aRadioUrl.HasParam("uart-init-deassert"))
         {
             // When flow control is disabled, deassert DTR and RTS on init
-#ifndef __APPLE__
-            int flags;
-#endif
+            int flags = TIOCM_DTR | TIOCM_RTS;
 
-            tios.c_cflag &= ~(CRTSCTS);
-
-#ifndef __APPLE__
-            // Deassert DTR and RTS
-            flags = TIOCM_DTR | TIOCM_RTS;
-            VerifyOrExit(ioctl(fd, TIOCMBIC, &flags) != -1, perror("tiocmbic"));
-#endif
+            VerifyOrExit((rval = ioctl(fd, TIOCMBIC, &flags)) == 0, perror("tiocmbic"));
         }
 
         VerifyOrExit((rval = cfsetspeed(&tios, static_cast<speed_t>(speed))) == 0, perror("cfsetspeed"));