[PATCH] USB: g_file_storage: export "stall" parameter

This patch changes the g_file_storage driver to make the "stall" module
parameter generally available; currently it is available only if the
testing version of the module has been configured.  It also fixes a typo
in a comment -- thanks, Pat!

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>