usb: usbtmc: Add ioctls for trigger, EOM bit and TermChar

- 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.

- add USBTMC_IOCTL_EOM_ENABLE to specify EOM bit for next write() call. Sets Bit 0 of field 'bmTransferAttributes' of DEV_DEP_MSG_OUT Bulk-OUT Header.

Allows fine grained control over end of message handling on a per file descriptor basis.

- add USBTMC_IOCTL_CONFIG_TERMCHAR to control TermChar handling for next read(). Controls field 'TermChar' and Bit 1 of field 'bmTransferAttributes' of REQUEST_DEV_DEP_MSG_IN BULK-OUT header.

Allows enabling/disabling of terminating a read on reception of term_char individually for each read request.