qseecom: optimize qseecom_unregister_listener operation

Make change to search listener svc entry just once and then use
this svc entry pointer directly when unmapping dma buffer address.
This change can also avoid calling a might sleep function in
qseecom_vaddr_unmap() within irq disabled context.

Change-Id: Ie7c18ce37c60558ac20ab784710c11f8350723a3
Signed-off-by: Zhen Kong <zkong@codeaurora.org>