gve: Allow pageflips on larger pages

Half pages are just used for small enough packets. This change allows
this to also apply for systems with pages larger than 4 KB.

Fixes: 02b0e0c1 ("gve: Rx Buffer Recycling")
Signed-off-by: Jordan Kim <jrkim@google.com>
Signed-off-by: Jeroen de Borst <jeroendb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>