rdma: Don't allocate sparse array

rdma: Don't allocate sparse array

The addition of driver QP type with index 0xFF caused to the following
clang compilation error:

res.c:152:10: warning: result of comparison of constant 256 with expression of type 'uint8_t' (aka 'unsigned char') is always true [-Wtautological-constant-out-of-range-compare]
        if (idx < ARRAY_SIZE(qp_types_str) && qp_types_str[idx])
            ~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~

Instead of allocating very sparse array, simply create separate check
for the driver QP type.

Fixes: 39307384cea7 ("rdma: Add driver QP type string")
Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>