Staging: rtl8192u: Do not DMA on the stack

Fix error "doing DMA on the stack" by using kzalloc for buffer
allocation.
Issue found by smatch.

Signed-off-by: Ksenija Stanojevic <ksenija.stanojevic@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>