wishbone_pkg: fix ghdl compile errors due to loop over length of a variable

Variable lengths might change; standard forbids length in a loop.
Fixes:
  wishbone_pkg.vhd:1576:18: universal integer bound must be numeric literal or attribute
  wishbone_pkg.vhd:1613:18: universal integer bound must be numeric literal or attribute
  wishbone_pkg.vhd:1734:30: universal integer bound must be numeric literal or attribute
  wishbone_pkg.vhd:1771:16: universal integer bound must be numeric literal or attribute
  wishbone_pkg.vhd:1806:16: universal integer bound must be numeric literal or attribute
  wishbone_pkg.vhd:1817:16: universal integer bound must be numeric literal or attribute