diag: Free the mask update client buf in exit case

Free the diag mask update client buf in diag_mask_exit()
during memory device mode application exit.

Change-Id: I8e747066924c993170af952f9bb9893c84c96445
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>