staging: goldfish: switch from spinlock to mutex

Use a mutex instead of a spinlock in goldfish_nand.c, as suggested by
the TODO list.

Signed-off-by: Kristina Martšenko <kristina.martsenko@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>