[PATCH 4/6] arm/duart: make dt_uart_init() compile with -Wwrite-strings

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

GCC complains:

  drivers/char/arm-uart.c: In function 'dt_uart_init':
  drivers/char/arm-uart.c:81:17: error: assignment discards 'const' qualifier 
from pointer target type [-Werror=discarded-qualifiers]
     81 |         options = "";
        |                 ^

The problem is using the options string for both splitting opt_duart, and to
hold a token "" for no options.

Use two variables; one mutable one one const.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>
CC: Michal Orzel <michal.orzel@xxxxxxx>
CC: Roberto Bagnara <roberto.bagnara@xxxxxxxxxxx>
CC: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
---
 xen/drivers/char/arm-uart.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/xen/drivers/char/arm-uart.c b/xen/drivers/char/arm-uart.c
index 8098a968c285..91f13a41368d 100644
--- a/xen/drivers/char/arm-uart.c
+++ b/xen/drivers/char/arm-uart.c
@@ -42,7 +42,8 @@ static void __init dt_uart_init(void)
     struct dt_device_node *dev;
     int ret;
     const char *devpath = opt_dtuart;
-    char *options;
+    const char *options;
+    char *split;
 
     if ( !console_has("dtuart") )
         return; /* Not for us */
@@ -74,9 +75,12 @@ static void __init dt_uart_init(void)
         return;
     }
 
-    options = strchr(opt_dtuart, ':');
-    if ( options != NULL )
-        *(options++) = '\0';
+    split = strchr(opt_dtuart, ':');
+    if ( split )
+    {
+        split[0] = '\0';
+        options = split + 1;
+    }
     else
         options = "";
 
-- 
2.30.2