Unify the codepaths for WELS_GET_PERFIX_BITS at the second place

This changes a second use of the macro which was missed in
60e0255677.

This avoids warnings about unused local variables in 64 bit
builds in MSVC.
Merge pull request #247 from mstorsjo/simplify-missed-inline-asm-macro

Unify the codepaths for WELS_GET_PERFIX_BITS at the second place