ixgbe sysctl hardware defaults

Added hw.ix.flow_control which enables the default flow_control of all ix
interfaces to be set in loader.conf.

Added hw.ix.advertise_speed which enables the default advertised_speed of
all ix interfaces to be set in loader.conf.

Made enable_aim device independent based on hw.ix.enable_aim default.

Reviewed by: erj
MFC after: 1 week
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D5060