serial: max3100: Update uart_driver_registered on driver removal

The removal of the last MAX3100 device triggers the removal of
the driver. However, code doesn't update the respective global
variable and after insmod — rmmod — insmod cycle the kernel
oopses:

  max3100 spi-PRP0001:01: max3100_probe: adding port 0
  BUG: kernel NULL pointer dereference, address: 0000000000000408
  ...
  RIP: 0010:serial_core_register_port+0xa0/0x840
  ...
   max3100_probe+0x1b6/0x280 [max3100]
   spi_probe+0x8d/0xb0

Update the actual state so next time UART driver will be registered
again.

Hugo also noticed, that the error path in the probe also affected
by having the variable set, and not cleared. Instead of clearing it
move the assignment after the successfull uart_register_driver() call.

Fixes: 7831d56b ("tty: MAX3100")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20240402195306.269276-3-andriy.shevchenko@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/max3100.c

@@ -749,13 +749,14 @@ static int max3100_probe(struct spi_device *spi)
 	mutex_lock(&max3100s_lock);
 
 	if (!uart_driver_registered) {
-		uart_driver_registered = 1;
 		retval = uart_register_driver(&max3100_uart_driver);
 		if (retval) {
 			printk(KERN_ERR "Couldn't register max3100 uart driver\n");
 			mutex_unlock(&max3100s_lock);
 			return retval;
 		}
+
+		uart_driver_registered = 1;
 	}
 
 	for (i = 0; i < MAX_MAX3100; i++)
@@ -841,6 +842,7 @@ static void max3100_remove(struct spi_device *spi)
 		}
 	pr_debug("removing max3100 driver\n");
 	uart_unregister_driver(&max3100_uart_driver);
+	uart_driver_registered = 0;
 
 	mutex_unlock(&max3100s_lock);
 }