UPSTREAM: staging: gdm72xx: use kzalloc to allocate usb_tx structure

UPSTREAM: staging: gdm72xx: use kzalloc to allocate usb_tx structure

the code under alloc_tx_struct does the allocation of usb_tx structure
using kmalloc, and memsets the allocated pointer, instead we can
directly use kzalloc so that the allocated memory is set with
zeros

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 07ad99c9e32173852997523eb920950902563941)

Change-Id: I623dd5512b116924c7a5fbaefbbd26296a14cdab
Reviewed-on: https://gerrit.chromium.org/gerrit/32489
Reviewed-by: Olof Johansson <olofj@chromium.org>
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>