fm10k: remove fm10k_get_reta_size from namespace

The function is only used in fm10k_ethtool.c, so make it static.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>