staging: ti dspbridge: avoid possible NULL dereference panic

When dsp_notifications array is received from user,
dspbridge verifies the array has valid pointers
and dsp_notification structures. However, these
structures contain pointers that need to be
checked for valid handles.

Signed-off-by: Ernesto Ramos <ernesto@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>