usb_dev: Declare ctrl buffer inside qiprog_handle_control_request

The control buffer should not be visible outside of
qiprog_handle_control_request.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
usb_dev: Add preliminary infrastructure for bulk-reading chip

This is the minimum infrastructure needed to handle bulk reading of the
flash chip in firmware. On the VultureProg test hadware, it achieves bulk
reading with insignificant deadtime on the LPC bus.

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