Clean up RE code in sceChnnlsv.{cpp,h}

Name some functions.
Make functions more consistent (same prototypes, order, structure for eg.).
Make functions more readable.
Avoid redundancies that IDA has pushed in there.
Remove length checks where they would be otherwise handled anyway.
Remove about 50 for loops. Instead opt for memset, memcpy and memxor where possible.

Might want to look over in case I made a typo somewhere.