disk: part_dos: fix part_get_info_extended() function

The check in part_get_info_extended() for a successful partition
searching misses a condition for extended partition. In case of
(ext_part_sector == 0), we should anyway mark the partition as found,
even if it's an extended partition, i.e. (is_extended(pt->sys_ind) == 0).
Otherwise, the extended partition (type 0x0f) will never be identified,
and the following recursive call to part_get_info_extended() will get a
wrong 'part_num' and 'which_part' parameter.  In the end, all those
partitions in extended table will not be identified.

Let's add the missing OR condition of (ext_part_sector == 0) for
is_extended() check to fix the problem.

The issue is discovered by running fastboot flash to an extended
partition on eMMC.

  $ fastboot flash mmcsda5 cache.img
  target reported max download size of 536870912 bytes
  sending 'mmcsda5' (18796 KB)...
  OKAY [  2.144s]
  writing 'mmcsda5'...
  FAILED (remote: cannot find partition)
  finished. total time: 2.261s

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>