PCI: remove redundant __msi_set_enable()

We have the 'pos' of the MSI capability at all locations which call
msi_set_enable(), so pass it to msi_set_enable() instead of making it
find the capability every time.

Reviewed-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>