ixl(4): Add tunable to override Flow Control settings

Add flow_control to hw.ixl tunables tree to let override
initial flow control configuration for all interfaces.
Keep using configuration set by NVM by default.

Reviewed by: erj@, gallatin@
Tested by: gowtham.kumar.ks_intel.com
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D29338