drivers: console: Fix compilation error in uart_console.c

Fix compilation error when CONFIG_USB_UART_CONSOLE is not enabled:

drivers/console/uart_console.c: In function 'uart_console_init':
drivers/console/uart_console.c:497:12: error: unused variable 'dtr' [-Werror=unused-variable]
  uint32_t  dtr = 0;

Change-Id: I592724150f86199f6a57354c3246197db2ac7a01
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>