mps: Use 64-bit chain structures

According to Broadcom, mixing 64-bit SGEs with 32-bit chain entries can
lead to IOC Fault code 0x40000d04. This fault code has been observed to
suddenly increase on certain machines when the OCA firmware images are
deployed. The hardware interprets all elements of a 64-bit SGE, even
ones marked as 32-bit. Depending on the other bits, this will just work,
but sometimes generate the above fault. Broadcom recommends this
practice, and the Linux and NetBSD drivers follow it.

Rework the chaining code to use MPI2_SGE_CHAIN64 instead of
MPI2_SGE_CHAIN32. Adjust MPS_SGC_SIZE from 8 to 12 to match the size of
the new structure. Flag the structure as being 64-bits now. Since
MPS_SGE64_SIZE and MPS_SGC_SIZE are the same now, mps_push_sge could be
simplified (after the same fashion of mpr). The different number of
cases collapse to whether or not there's room for the segments and if
not we need a chain, however these changes haven't been made yet as the
current code handles those cases properly with the new defines.

Made chain_busaddr 64-bits, even though we ask for all allocations to be
below 4GB for this tag. Use it to set both parts of the CHAIN64 address
rather than baking the 4GB assumption. Add asserts around the allocation
to detect and BUSDMA bugs in allocation.

Remove asserts and associated comment in mpi_pre_fw_download and
mpi_pre_fw_upload. The code does not, it seems, depend on this
invariant. The mpr driver has similar code, no asserts and also doesn't
depend on this.

Adjust comments to reflect the updated size.

Sponsored by: Netflix
Reviewed by: scottl, mav
Differential Revision: https://reviews.freebsd.org/D34016