at91sam7: do not touch flash banks which belong to other targets

at91sam7_read_part_info() walks throw all flash banks following
current one.

I don't know why it has to do it at all (possibly for multi-bank
devices like SAM7S512), but if there is more than one target in JTAG
chain, this lookup can touch flash bank of another (possibly not
halted) target, which cause probe error and current command
execution abort.

[andreas.fritiofson@gmail.com]: Change to for-loop and reduce
indentation

Change-Id: Ide50e93578786e1250f7a0fd0e3d296247924814
Signed-off-by: Sergey A. Borshch <sb-sf@users.sourceforge.net>
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/2610
Tested-by: jenkins