remove empty byte truncation from stlink_fread

[gtalusan]

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

st-flash read out.bin 0x8000000

I expect the output to be 0x80000 bytes in size but I get something quite shorter. Appending the length to the command line also produces the same short output. Looking at the code, I see that the number of FF's or 00's is counted on each iteration of the read, but the file is truncated after all of the reads are done.

I think the writes should be shortened inside the loop, but it looks like honouring sl->q_len is the way to go.

This patch removes the erased byte counter, bumps up the read size to min(0x1800, flash page size) similar to the verify flash data function.

[gtalusan]

Also wonder if #308 is related.