software/bios/readline: Fix warnings if char is signed

When building with --cpu-type=mor1kx:

    litex/soc/software/bios/readline.c: In function 'readline':
    litex/soc/software/bios/readline.c:271:3: warning: case label value exceeds maximum value for type [-Wswitch-outside-range]
      271 |   case KEY_END:
	  |   ^~~~
    litex/soc/software/bios/readline.c:297:3: warning: case label value exceeds maximum value for type [-Wswitch-outside-range]
      297 |   case KEY_DEL:
	  |   ^~~~
    litex/soc/software/bios/readline.c:281:3: warning: case label value exceeds maximum value for type [-Wswitch-outside-range]
      281 |   case DEL:
	  |   ^~~~

The C standard does not specify the signedness of "char", hence this
depends on the implementation.  On e.g. RISC-V, "char" is unsigned, but
on OpenRISC, it is signed.

Fix this by making the "ichar" variable explicitly unsigned.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

litex/soc/software/bios/readline.c

@@ -190,7 +190,7 @@ int readline(char *buf, int len)
 	unsigned int eol_num = 0;
 	unsigned int wlen;
 	int insert = 1;
-	char ichar;
+	unsigned char ichar;
 
 #ifndef TERM_NO_COMPLETE
 	char tmp;