Initialize the whole message unit's DMA buffer to zero, this fixes a panic

Initialize the whole message unit's DMA buffer to zero, this fixes a panic
during boot when ARC1200 is being used with certain motherboard models.

This commit brings the driver to the same state of vendor's 1.20.00.16
release.  Many thanks to Areca for their continued support to FreeBSD.

Reported by:	Jirka Mikulas <jiri mikulas com>
Submitted by:	Erich Chen (Areca)
Obtained from:	ftp://ftp.areca.com.tw/RaidCards/AP_Drivers/FreeBSD/DRIVER/SourceCode/arcmsr-freebsd-1.20.00.16-91010.zip
MFC after:	3 days