tqm5200: Correct comment and code in post_hotkeys_pressed.

This fixes the code and the comment according to the original intent of
doing an intensive memory test when PSC6_3 is pulled low on the STK52xx.
Notably PORT_CONFIG will be overridden with this correct code now,
so beware.

The original code only worked by coincidence depending on the PORT_CONFIG
setting from the header file.  The new code was tested to ensure that the
(undocumented) memory test still works on the STK52x.

Signed-off-by: Detlev Zundel <dzu@denx.de>
CC: Martin Krause <Martin.Krause@tqs.de>

Minor white-space cleanup.
Signed-off-by: Wolfgang Denk <wd@denx.de>