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>
usb_master: Fix uninitialized variable when resubmitting transfers

In the async callback, we are assuming that 'ret' is initialized to
QIPROG_SUCCESS before checking for errors, however it was never
initialized. This caused the transfer to not be resubmitted, without
any error message. The transfer was then removed from the list if
active transfers, and the loop returned without having transmitted the
complete dataset.
Initialize 'ret' to QIPROG_SUCCESS before any error checking.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
qiprog: Support SST49LF040B

Signed-off-by: Peter Stuge <peter@stuge.se>
libqiprog: Update documentation to reflect latest developments

A few changes have been made to the internal structure of libqiprog
which were not reflected in the documentation:
 * removal of qiprog_set_address()
 * document qiprog_read/write()
 * document asymmetry between write8/16/32() and write()

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
NFM: Temporarily re-enable USB bulk timing code