Update isl6422.c: Allow loading by setting attach function as GPL

[pjacobs-eu]

The ISL6422 LNB power chip refuses to work in modern kernels due to a restriction. By using EXPORT_SYMBOL_GPL, the kernel module loads correctly once again.

Also see: https://lore.kernel.org/lkml/20230731083806.453036-6-hch@lst.de/

[crazycat69]

thanks, merged