linux_usbfs: Update MAX_ISO_BUFFER_LENGTH

linux_usbfs: Update MAX_ISO_BUFFER_LENGTH

Newer kernels have raised the maximum length of individual ISO
packets and URBs. There's no easy way to detect the limit, so we
will define MAX_ISO_BUFFER_LENGTH as the largest known limit.

If a user runs this on an earlier kernel and submits an ISO transfer
that is too large, they will receive LIBUSB_ERROR_INVALID_PARAM.

The documentation has been updated to note this behavior, under
"Considerations for isochronous transfers".

Closes #23

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>