usb: usbtmc: Add ioctl for trigger

add USBTMC488_IOCTL_TRIGGER to send TRIGGER Bulk-OUT header
according to Subclass USB488 Specification

The usbtmc trigger command is equivalent to the IEEE 488 GET (Group
Execute Trigger) action. While the "*TRG" command can be sent as
data to perform the same operation, in some situations an instrument
will be busy and unable to process the data immediately in which
case the USBTMC488_IOCTL_TRIGGER can be used to trigger the
instrument with lower latency.

Reviewed-by: Steve Bayless <steve_bayless@keysight.com>
Tested-by: Dave Penkler <dpenkler@gmail.com>
Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Signed-off-by: Guido Kiener <guido.kiener@rohde-schwarz.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>