fm10k: remove extraneous variable definition in fm10k_ethtool.c

We don't need to typecast a u8 * into a char *, so just remove the extra
variable.

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>