gve: fix unused variable/label warnings

On unusual page sizes, we get harmless warnings:

drivers/net/ethernet/google/gve/gve_rx.c:283:6: error: unused variable 'pagecount' [-Werror,-Wunused-variable]
drivers/net/ethernet/google/gve/gve_rx.c:336:1: error: unused label 'have_skb' [-Werror,-Wunused-label]

Change the preprocessor #if to regular if() to avoid this.

Fixes: f5cedc84

 ("gve: Add transmit and receive support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>