diag: Add notification that diag supports feature mask

diag does not currently set the bit in the feature mask denoting
that APSS diag supports the sending/receiving of the feature mask.
Without this, the diag on the peripiherals are unsure whether APSS
diag supports this feature. Make modifications to set the bit in
the feature mask so that diag on the peripherals will know whether
APSS diag supports feature mask sending and receiving.

Also, the function diag_dci_exit is not being called. Add appropriate
calls to diag_dci_exit.

Change-Id: I7cb67a96d6f10af1702bab4e0458bce55e437678
Signed-off-by: Dixon Peterson <dixonp@codeaurora.org>