batman-adv: Drop initialization of flexible ethtool_link_ksettings

The commit 94dfc73e ("treewide: uapi: Replace zero-length arrays with
flexible-array members") changed various structures from using 0-length
arrays to flexible arrays

  net/batman-adv/bat_v_elp.c: note: in included file:
  ./include/linux/ethtool.h:148:38: warning: nested flexible array
  net/batman-adv/bat_v_elp.c:128:9: warning: using sizeof on a flexible structure

In theory, this could be worked around by using {} as initializer for the
variable on the stack. But this variable doesn't has to be initialized at
all by the caller of __ethtool_get_link_ksettings - everything will be
initialized by the callee when no error occurs.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>