[IA64] Fix breakage in simscsi.c

arch/ia64/hp/sim/simscsi.c: In function `simscsi_sg_readwrite':
arch/ia64/hp/sim/simscsi.c:154: error: structure has no member named `buffer'
arch/ia64/hp/sim/simscsi.c: In function `simscsi_fillresult':
arch/ia64/hp/sim/simscsi.c:247: error: structure has no member named `buffer'

hch said:
>Just change it to access the request_buffer member instead.  buffer
>and request_buffer have been synonymous 99% of the time, and a driver
>never even wants to access buffer.

Signed-off-by: Tony Luck <tony.luck@intel.com>