usb_storage: Remove EHCI constraints

Now that the EHCI driver allocates its qTDs from the heap, the MSC driver is
only limited by the SCSI commands it uses.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Ilya Yanok <ilya.yanok@cogentembedded.com>
Cc: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>