atmel-ssc: fix misuse of dev_dbg when requested ssc instance is not found

The ssc pointer is not valid when the id is not found in the list.
Convert the message from a debug one into an error message and avoid
dereferencing the bad pointer.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Huang Weiyi <weiyi.huang@gmail.com>
Acked-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>