pcmcia: use mutex for dynid lock

Even though we weren't calling a blocking function within the dynid
spinlock, we do not need a spinlock here but can and should be using
a mutex.

Reported-by: Jiri Slaby <jirislaby@gmail.com>
Tested-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>