arm64: fix db_read_bytes() for size == 8

[mhorne]

Summary

There is a mistake in the cast, resulting in a truncated read to tmp64.

Reported by: dfr
PR: 276406

[mhorne]

On a second look, the native-sized reads were not an optimization but rather to enable reading from device memory, per a67687fcd8f53.

So, just fix the cast.

[dfr]

Looks good, thanks!

[mmel]

Thanks fixing my bugs..

[emaste]

Seems like there ought to be a comment before the if size == 8 / else if / else if block.

[jhb]

sys/arm64/arm64/db_interface.c

134

I think const uint64_t * would be clearer here, and similarly for the other casts below.