asix-sigma: more u16 sample memory access nits (timestamps, values)

asix-sigma: more u16 sample memory access nits (timestamps, values)

Further "flatten" the DRAM layout's declaration for sample data. Declare
timestamps and sample data as uint16_t, keep accessing them via endianess
aware conversion routines. Accessing a larger integer in smaller quantities
is perfectly fine, the inverse direction would be problematic.