jtag: fix support for really long scans

jtag: fix support for really long scans

When programming large FPGAs the generated SVF files might contain really
long SDR scans. They won't fit in the 1MiB jtag scan page at all, so in
this case the allocated page needs to be bigger. The current code was
silently corrupting memory.

One particular example was sent by Volter targetting XC3S4000. It has an
SDR 11316992 bits long, that is 1414624 bytes.

Change-Id: I39f18d7e0654f2dbdf37df58c837c9ec1fb2aa2a
Reported-by: "Voltner, Jiří" <j.voltner@era.aero>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1792
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>