net/e1000: clean LTO warnings

During LTO build compiler reports some 'false positive' warnings about
variables being possibly used uninitialized.  This patch silences these
warnings.

Exemplary compiler warning to suppress (with LTO enabled):
error: ‘link’ may be used uninitialized in this function
[-Werror=maybe-uninitialized]
  if (link) {

Signed-off-by: Andrzej Ostruszka <aostruszka@marvell.com>