drivers: console: uart_mcumgr: Skip reading FIFO during setup

Drops calling the UART FIFO read function during the setup
function (when not in async mode) which could cause issues on
some devices since this function is not called in an ISR.

(cherry picked from commit 1b364c1)

Original-Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
GitOrigin-RevId: 1b364c1
Change-Id: I4e4ab6e289662614b63b64b9707c92f3bbc8ad78
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5062256
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Al Semjonovs <asemjonovs@google.com>
Tested-by: Al Semjonovs <asemjonovs@google.com>
Commit-Queue: Al Semjonovs <asemjonovs@google.com>

drivers/console/uart_mcumgr.c

@@ -228,16 +228,9 @@ static void uart_mcumgr_setup(const struct device *uart)
 #else
 static void uart_mcumgr_setup(const struct device *uart)
 {
-	uint8_t c;
-
 	uart_irq_rx_disable(uart);
 	uart_irq_tx_disable(uart);
 
-	/* Drain the fifo */
-	while (uart_fifo_read(uart, &c, 1)) {
-		continue;
-	}
-
 	uart_irq_callback_set(uart, uart_mcumgr_isr);
 
 	uart_irq_rx_enable(uart);