staging: ti-st: fix kim platform device id

Platform devices tend to have id as -1 when only 1 device exist and
a value >=0 when multiple devices exist, since we plan to store all
these platform devices if multiple exist, there was a requirement to
have id to be >=0.
The patch fixes this problem.

Signed-off-by: Pavan Savoy <pavan_savoy@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>