scsi: Have scsi_init_dev_desc_priv() use memset

When we do not have CONFIG_BOUNCE_BUFFER enabled, inside of
scsi_init_dev_desc_priv we never set the 'bb' field to false, we only
initialize it to true when CONFIG_BOUNCE_BUFFER is set. Given that we
have a number of other fields here we had been explicitly setting to
zero, change to first calling memset to clear the struct and then
initialize only the fields that need non-zero default values.

Addresses-Coverity-ID: 467407 ("Uninitialized variables (UNINIT)")
Fixes: 81bd22e9 ("rockchip: block: blk-uclass: add bounce buffer flag to blk_desc")
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>