staging: line6: use smallest iso ep packet size

The POD HD300 isochronous endpoints have different max packet sizes for
read and write.  Using the read endpoint max packet size may be too
large for the write endpoint.  Instead we should use the minimum of both
endpoints to be sure the size is acceptable.

In theory we could decouple read and write packet sizes but the driver
currently uses a single size which I chose not to mess with since other
features like software monitoring may depend on a single packet size for
both endpoints.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>