RDMA/mlx5: Fix stack out-of-bounds read in cc_params debugfs

get_param() reads a congestion parameter as a u32 but formats it with the
signed "%d" into an 11-byte stack buffer. A value with bit 31 set, such as
0x80000000, renders as "-2147483648\n" whose full length is 12. snprintf()
stores only 11 bytes yet returns 12, so simple_read_from_buffer() treats 12
bytes as valid and reads one byte past lbuf[].

Size the buffer for the widest unsigned decimal, format with "%u" to match
the u32, and use scnprintf() so the length passed to
simple_read_from_buffer() reflects the bytes actually stored.

Fixes: 4a2da0b8c0782 ("IB/mlx5: Add debug control parameters for congestion control")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>