ice: refactor FC functions

Create a helper function for configuring requested flow control so that it
can be utilized by other functions looking to configure flow control
settings. Utilize the existing helper ice_copy_phy_caps_to_cfg() to copy a
PHY capability to configuration instead duplicating the code for it.

Signed-off-by: Paul Greenwalt <paul.greenwalt@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>