libqiprog: Add separate read and write pointers to dev->addr

As agreed with Peter, read and write operations should not be
interdependent, and thus they will use distinct pointers. These pointers
should automatically be reset to addr.start after exhausting the range.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
libqiprog: Remove set_address() from the API (FTBFS)

set_address() is an artifact of the USB protocol, and after discussing it
with Peter, we agreed to remove it from the API. set_address() will
continue to be part of the usb driver, albeit hidden from the rest of the
world.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>