ath6kl: fix memory leak in ath6kl_fwlog_block_read()

If, in
drivers/net/wireless/ath/ath6kl/debug.c::ath6kl_fwlog_block_read(),
the call to wait_for_completion_interruptible() returns -ERESTARTSYS
then we'll return without freeing the (as yet unused) memory we
allocated for 'buf' - thus leaking it.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>