service/audren_u: Handle variadic command buffers in GetWorkBufferSize()

Also introduced in REV5 was a variable-size audio command buffer. This
also affects how the size of the work buffer should be determined, so we
can add handling for this as well.

Thankfully, no other alterations were made to how the work buffer size
is calculated in 7.0.0-8.0.0. There were indeed changes made to to how
some of the actual audio commands are generated though (particularly in
REV7), however they don't apply here.