usb_master: Use libusb_host_ctx when scanning for devices

struct qiprog_context.libusb_host_ctx is passed to libusb_init()
in qiprog_init() and the same context must be passed also to
libusb_get_device_list() when scanning for devices in usb_master.c.

Signed-off-by: Peter Stuge <peter@stuge.se>
qiprog: Say that we are writing flash chip on write operation

Copy-paste typo said "Attempting to read flash chip". Fix that.

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