staging: gasket: interrupt: refactor PCI MSIX-specific handler code

Split interrupt handler into PCI MSIX-specific and generic functions,
for adding non-MSIX handlers in the future.  Move MSIX init code
together,, out of generic init path.

Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>