dmi: fix date handling in dmi_get_year()

Year parsing in dmi_get_year() had the following two bugs.

* "00" is treated as invalid instead of 2000 because zero return from
  simple_strtoul() is treated as error.

* "0N" where N >= 8 is treated as invalid of 200N because the leading
  0 is considered to specify octal.

Fix the above two bugs by using endptr to detect invalid number and
forcing decimal.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>