infiniband: mlx5: avoid a compile-time warning

The return type of find_first_bit() is architecture specific,
on ARM it is 'unsigned int', while the asm-generic code used
on x86 and a lot of other architectures returns 'unsigned long'.

When building the mlx5 driver on ARM, we get a warning about
this:

infiniband/hw/mlx5/mem.c: In function 'mlx5_ib_cont_pages':
infiniband/hw/mlx5/mem.c:84:143: warning: comparison of distinct pointer types lacks a cast
     m = min(m, find_first_bit(&tmp, sizeof(tmp)));

This patch changes the driver to use min_t to make it behave
the same way on all architectures.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>