Staging: ti-st: Update TODO

An important TODO was to remove global references in TI-ST driver,
thereby providing a context to the driver.
This should also serve as a small step in removal of the single device
limit.

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