Restore SIOCGI2C functionality to ixgbe

When ixgbe was converted to iflib, it lost the SIOCGI2C support
that allows ifconfig to print SFP state, optical light levels, etc.
Restore this by plugging in to the ifdi_i2c_req iflib method. Note
that the sanity checking on dev_addr that used to be done in ixgbe is
now done in iflib.

Reviewed by: erj, Matthew Macy <mmacy@mattmacy.io>
Sponsored by: Netflix