mlx5: Remove uninitialized use of key in mlx5_core_create_mkey

Clang warns:

../drivers/net/ethernet/mellanox/mlx5/core/mr.c:63:21: warning: variable
'key' is uninitialized when used here [-Wuninitialized]
                      mkey_index, key, mkey->key);
                                  ^~~
../drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.h:54:6: note:
expanded from macro 'mlx5_core_dbg'
                 ##__VA_ARGS__)
                   ^~~~~~~~~~~
../include/linux/dev_printk.h:114:39: note: expanded from macro
'dev_dbg'
        dynamic_dev_dbg(dev, dev_fmt(fmt), ##__VA_ARGS__)
                                             ^~~~~~~~~~~
../include/linux/dynamic_debug.h:158:19: note: expanded from macro
'dynamic_dev_dbg'
                           dev, fmt, ##__VA_ARGS__)
                                       ^~~~~~~~~~~
../include/linux/dynamic_debug.h:143:56: note: expanded from macro
'_dynamic_func_call'
        __dynamic_func_call(__UNIQUE_ID(ddebug), fmt, func, ##__VA_ARGS__)
                                                              ^~~~~~~~~~~
../include/linux/dynamic_debug.h:125:15: note: expanded from macro
'__dynamic_func_call'
                func(&id, ##__VA_ARGS__);               \
                            ^~~~~~~~~~~
../drivers/net/ethernet/mellanox/mlx5/core/mr.c:47:8: note: initialize
the variable 'key' to silence this warning
        u8 key;
              ^
               = '\0'
1 warning generated.

key's initialization was removed in commit fc6a9f86 ("{IB,net}/mlx5:
Assign mkey variant in mlx5_ib only") but its use was not fully removed.
Remove it now so that there is no more warning.

Fixes: fc6a9f86 ("{IB,net}/mlx5: Assign mkey variant in mlx5_ib only")
Link: https://github.com/ClangBuiltLinux/linux/issues/932


Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>