[SCSI] arcmsr: fix message allocation

arcmsr_iop_message_xfer() is called from atomic context under the
queuecommand scsi_host_template handler. James Bottomley pointed out
that the current GFP_KERNEL|GFP_DMA flags are wrong: firstly we are in
atomic context, secondly this memory is not used for DMA.
Also removed some unneeded casts.

Signed-off-by: Daniel Drake <dsd@gentoo.org>
Cc: Nick Cheng <nick.cheng@areca.com.tw>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>