mlx4_core: Fix warning from min()

Recent cpumask changes changed num_possible_cpus() from returning an int
to returning an unsigned int.  This means that doing

    min(num_possible_cpus(), <int expression>)

now produces a warning like

    drivers/net/mlx4/main.c: In function 'mlx4_enable_msi_x':
    drivers/net/mlx4/main.c:915: warning: comparison of distinct pointer types lacks a cast

Fix this by using min_t(int, ...).

Signed-off-by: Roland Dreier <rolandd@cisco.com>