drivers/block/DAC960: fix -Wuninitialized warning

Set CommandMailbox with memset before use it. Fix for:

drivers/block/DAC960.c: In function ‘DAC960_V1_EnableMemoryMailboxInterface’:
arch/x86/include/asm/io.h:61:1: warning: ‘CommandMailbox.Bytes[12]’
 may be used uninitialized in this function [-Wuninitialized]
drivers/block/DAC960.c:1175:30: note: ‘CommandMailbox.Bytes[12]’
 was declared here

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>