Clean up MAC address conversion in HLE

Not planning to implement net, but this fixes a potential crash / buffer overflow, and implements a commonly reported unimplemented function.

sceNetEtherStrton() has been reported 133,675 times.  99.89% of these are Patapon 2 (which calls it at startup.)  There's also Metal Gear Solid: Peace Walker and Bubble Bobble Evolution, and then a few with a report or two.

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