clamp gdb memory reads to 0x1800

[gtalusan]

Similar to #371.

I'm trying to backup the flash memory on an EMW3165 with an eBay ST-Link v2 using the following command:

$ gdb
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(gdb) target extended-remote :4242
Remote debugging using :4242
0x00000000 in ?? ()
(gdb) dump binary memory out.bin 0x8000000 0x8080000

st-util -v99 shows the USB transfer erring out with a -6. The gdbserver output shows that 0x1ffff chunks of bytes are being requested but this exceeds the magic 0x1800 in the code.

This patch will return at most 0x1800 bytes. In my testing, it looks like gdb generates the same output file as st-flash does after #371.