FMC: fix locking in sample chardev driver

The fmc-chardev module was over-locking, by calling misc
register/unregister while holding its spinlock.  This reported a
"scheduling while atomic" error.

Since the misc driver already serializes operations internally, this
commit downgrades the fmc-chardev lock to just cover its own list.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>