xscale: stackframe corruption bugfix

Resolve a "FIX" comment; yes that was superfluous given that the
JTAG core does that check by default.  It was also buggy since it
wrote to a stack frame that went away before the write happened!!

Other fixes:  remove pointless malloc(); zero-init scan_field_t
values wherever they appear; whitespace scrub; spelling fix.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>