usb: usbtmc: Support Read Status Byte with SRQ per file

usb: usbtmc: Support Read Status Byte with SRQ per file

Add 'struct usbtmc_file_data' for each file handle to cache last
srq_byte (=Status Byte with SRQ) received by usbtmc_interrupt(..)

usbtmc488_ioctl_read_stb returns cached srq_byte when available for
each file handle to avoid race conditions of concurrent applications.

SRQ now sets EPOLLPRI instead of EPOLLIN since EPOLLIN is now reserved
for asynchronous reads

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