mlx5: minor fixes (mainly avoidance of hidden casts)

There were many places where parameters which should be u8/u16 were
integer type.

Additionally, in 2 places, a check for a non-null pointer was added
before dereferencing the pointer (this is actually a bug fix).

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>