fm10k: remove needless initialization of size local variable

The local variable 'size' in fm10k_dfwd_add_station is initialized, but
is always re-assigned immediately before use. Remove this unnecessary
initialization.

This was detected by cppcheck and resolves the following warning
produced by that tool:

[fm10k_netdev.c:1466]: (style) Variable 'size' is assigned a value that is never used.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>