lib: utils/serial: Fix semihosting compile error using LLVM

We fix the following semihosting compile error observed using LLVM:
lib/utils/serial/semihosting.c:158:12: error: result of comparison of constant -1 with expression of type 'char' is always true [-Werror,-Wtautological-constant-out-of-range-compare]
                ret = ch > -1 ? ch : -1;
                      ~~ ^ ~~

Fixes: 7f09fba8

 ("lib: utils/serial: add semihosting support")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>