usb: gadget: storage: increase maximum storage num buffers

With a default size of 16kiB and with maximum of 32
buffers, we can transfer up to 512kiB, however Linux
can transfer up to 1MiB in a single mass storage
block transfer to USB3 storage devices.

Because of this, 1MiB block transfers end up being
slower than 512kiB block transfers. Let's increase
maximum number of storage buffers to a ridiculous
amount (256) so that anybody wanting to test maximum
achievable throughput can do so.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>