tools: value checks in rkcommon_check_params()

Building with -Wtype-limits yields

tools/rkcommon.c: In function ‘rkcommon_check_params’:
tools/rkcommon.c:158:27: warning: comparison of
unsigned expression < 0 is always false [-Wtype-limits]
  158 |  if (spl_params.init_size < 0)
      |                           ^
tools/rkcommon.c:165:28: warning: comparison of
unsigned expression < 0 is always false [-Wtype-limits]
  165 |   if (spl_params.boot_size < 0)
      |

Fix the value checks.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>