acpi: use constants as strncpy limit

gcc is not smart enough to figure out length was validated before use as
strncpy limit, resulting in this warning:

inlined from ‘virt_set_oem_table_id’ at ../../hw/arm/virt.c:2197:5:
/usr/include/aarch64-linux-gnu/bits/string_fortified.h:106:10: error:
‘__builtin_strncpy’ specified bound depends on the length of the
source argument [-Werror=stringop-overflow=]

Simplify things by using a constant limit instead.

Fixes: 97fc5d507fca ("acpi: Permit OEM ID and OEM table ID fields to be changed")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>