usb: bdc: driver runs out of buffer descriptors on large ADB transfers

Version v1.0.40 of the Android host ADB software increased maximum
transfer sizes from 256K to 1M. Since the STB ADB gadget driver
requests only 16K at a time, the BDC driver ran out of buffer
descriptors (BDs) if the queuing happens faster than the incoming
16K transfers. This issue is fixed by doubling the number of BDs
that can be queued so that the entire 1M request can be queued
without running out of buffers.

Signed-off-by: Al Cooper <alcooperx@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>