igb: Enable SR-IOV after reinit

Enabling SR-IOV causes the virtual functions to make requests to the
PF via the mailbox. Notably, E1000_VF_RESET request will happen during
the initialization of the VF. However, unless the reinit is done, the
VMMB interrupt, which delivers mailbox interrupt from VF to PF will be
kept masked and such requests will be silently ignored.

Enable SR-IOV at the very end of the procedure to configure the device
for SR-IOV so that the PF is configured properly for SR-IOV when a VF is
activated.

Fixes: fa44f2f1

 ("igb: Enable SR-IOV configuration via PCI sysfs interface")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Tested-by: Marek Szlosek <marek.szlosek@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>