lib: sign conflict in wake_coldboot_harts()

Compiling wake_coldboot_harts() with GCC 11 and -Wextra yields:

lib/sbi/sbi_init.c:208:27:
error: comparison of integer expressions of different signedness:
‘int’ and ‘u32’ {aka ‘unsigned int’} [-Werror=sign-compare]
  208 |         for (int i = 0; i <= sbi_scratch_last_hartid(); i++) {
      |                           ^~

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Xiang W <wxjstz@126.com>