gve: Correct available tx qpl check

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>