net/mlx4: Use cpumask_available for eq->affinity_mask

Clang warns that the address of a pointer will always evaluated as true
in a boolean context:

drivers/net/ethernet/mellanox/mlx4/eq.c:243:11: warning: address of
array 'eq->affinity_mask' will always evaluate to 'true'
[-Wpointer-bool-conversion]
        if (!eq->affinity_mask || cpumask_empty(eq->affinity_mask))
            ~~~~~^~~~~~~~~~~~~
1 warning generated.

Use cpumask_available, introduced in commit f7e30f01 ("cpumask: Add
helper cpumask_available()"), which does the proper checking and avoids
this warning.

Link: https://github.com/ClangBuiltLinux/linux/issues/86


Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>