cmd: efidebug: fix -Werror=type-limits warning

Compiling with -Wtype-limits yields:

cmd/efidebug.c:968:32: error: comparison is always false due to limited
range of data type [-Werror=type-limits]
  968 |  if (*endp != '\0' || bootnext > 0xffff) {
      |

Remove the superfluous check.

Fixes: 59df7e7e ("cmd: add efidebug command")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>