[PATCH] 3c59x: correct rx_dropped counting

Only increment rx_dropped in case of lack of resources (i.e. not for
frames with errors).

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>