Staging: rc2860: return -EFAULT on copy_to_user errors

copy_to_user() returns the number of bytes remaining but we want to
return a negative error code.  This is in the ioctl handler and the
error code gets passed to userspace.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>