bluetooth: Fix unused var warning properly in rfcomm_sock_ioctl().

As Stephen Rothwell points out, we don't want 'sock' here but
rather we really do want 'sk'.

This local var is protected by all sorts of bluetooth debugging
kconfig vars, but BT_DBG() is just a straight pr_debug() call
which is unconditional.

pr_debug() evaluates it's args only if either DEBUG or
CONFIG_DYNAMIC_PRINTK_DEBUG is defined.

Solving this inside of the BT_DBG() macro is non-trivial since
it's varargs.  And these ifdefs are ugly.

So, just mark this 'sk' thing __maybe_unused and kill the ifdefs.

Signed-off-by: David S. Miller <davem@davemloft.net>