ANDROID: of: Fix build warnings

In commit d6cb004d80 (of: fix CONFIG_CMDLINE_EXTEND),
the types of some variables in early_init_dt_scan_chosen()
were modified, which results in build warnings.

This patch resets the unsigned long to an int, and re-adds
the const to the char*.

Change-Id: Ie60ae92b4552e453cf477dd83f42838b3f95975e
Signed-off-by: John Stultz <john.stultz@linaro.org>