staging: tidspbridge: Remove cfg_get_exec_file()

As the services directory is going to be removed, the cfg_get_exec_file
function has also to be removed.

This patch also avoids a possible NULL pointer dereference in function
cfg_get_exec_file(), when drv_datap is checked for NULL and then pass
drv_datap->base_img as argument to strlen().

Signed-off-by: Ivan Gomez Castellanos <ivan.gomez@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>