staging: bcm2835-camera: Convert spinlock to mutex in handle mapping code

The handle mapping code that converts context pointers to handles uses
a spinlock.  Since the btree implementation can sleep while allocating
memory, turning on several kernel debugging options will result in
errors in the log.

Since this code path is never called in atomic context, perhaps it's
better to just use a mutex.

Signed-off-by: Michael Zoran <mzoran@crowfest.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>