gve: Correct available tx qpl check

[ Upstream commit d03477ee10f4bc35d3573cf1823814378ef2dca2 ]

The qpl_map_size is rounded up to a multiple of sizeof(long), but the
number of qpls doesn't have to be.

Fixes: f5cedc84 ("gve: Add transmit and receive support")
Signed-off-by: Catherine Sullivan <csully@google.com>
Signed-off-by: Jeroen de Borst <jeroendb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>