atl2: prevent compiler memory access misordering

atl2: prevent compiler memory access misordering

While no problems have been observed in practice yet, modern compilers
may reorder memory access operations, and that could lead to problems
with memory-mapped I/O typically done by drivers. This patch prevents
any potentially problematic reordering by the compiler in the ATL2
driver.

In addition, this patch removes a number of gcc/clang warnings.