Fix ixl(4) compilation with PCI_IOV pre-r266974

stable/10 doesn't have the if_getdrvflags(9) KPI. Reference the field in the
structure directly if the __FreeBSD_version is < 1100022, so the driver can
be built with PCI_IOV support on stable/10, without backporting all of
r266974 (which requires additional changes due to projects/ifnet, etc)

Differential Revision: https://reviews.freebsd.org/D4759
Reviewed by: erj, sbruno
Sponsored by: EMC / Isilon Storage Division