efi: serial: Use correct EFI status type

int is not sufficient to hold and test the return from an EFI function
call. Use efi_status_t instead so that the test can work as expected.

This issue was found by Smatch.

Fixes: 275854baee ("efi: Add a serial driver")
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

drivers/serial/serial_efi.c

@@ -33,7 +33,7 @@ int serial_efi_setbrg(struct udevice *dev, int baudrate)
 
 static int serial_efi_get_key(struct serial_efi_priv *priv)
 {
-	int ret;
+	efi_status_t ret;
 
 	if (priv->have_key)
 		return 0;