usb storage: group dependent USB storage Kconfig entries together

Instead of explicit setting of USB_STORAGE dependency for every
underlying build entries, exploit if USB_STORAGE / endif block.

The change is a trivial non-functional cleanup, it shortens
the Kconfig file and it is expected to reduce zconf parser
workload a little.

Dependencies of USB_UAS build option are left aside deliberately.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>