s390-vregs.exp: Fix Tcl error after non-zero-pad patch

s390-vregs.exp: Fix Tcl error after non-zero-pad patch

s390-vregs.exp yields a Tcl error:

  ERROR: can't read "i": no such variable
      while executing
  "expr $a_high * ($i + 1) * $a_high "
      (procedure "hex128" line 2)
      invoked from within
  "hex128 $a_high $a_low $b_high $b_low"
  ...

This is a regression, caused by commit 30a254669b16b8 -- "Don't always
zero pad in print_*_chars".  That patch introduced a new procedure
"hex128" for formatting a 128-bit value as hex, but it accidentally moved
the calculation of the 128-bit value into that new procedure as well
instead of leaving it in the original context.  This is fixed.

gdb/testsuite/ChangeLog:

	* gdb.arch/s390-vregs.exp: Calculate parameters to hex128 in the
	calling context.
	(hex128): Drop erroneous calculation of parameters.