USB: g_multi kconfig: fix depends and help text

USB_G_MULTI uses block and net interface functions, so make it
depend on both of those.  Otherwise there are lots of build errors.

Fix USB_G_MULTI config help text typos and copy/paste error.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>