USB: usbtmc: Add USBTMC_IOCTL_GET_STB

This new ioctl reads the status byte (STB) from the device and returns
the STB unmodified to the application. The srq_asserted bit is not taken
into account and not changed.

This ioctl is useful to support non USBTMC-488 compliant devices.

Tested-by: Jian-Wei Wu <jian-wei_wu@keysight.com>
Reviewed-by: Guido Kiener <guido.kiener@rohde-schwarz.com>
Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Link: https://lore.kernel.org/r/20201215155621.9592-3-dpenkler@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>