Staging: most: fix doing DMA on stack

This patch fixes error "doing DMA on the stack" by using kzalloc
for buffer allocation.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>