usb: dfu: Reduce default bwPollTimeout value

The bwPollTimeout name is somewhat confusing as it refers to minimum
polling period. The bwPollTImeout is essentially a mechanism allowing
device to ratelimit DFU_GETSTATUS requests from host.

Setting bwPollTimeout to relatively high values effectively slows down
DFU download process. The slowdown is not observed if the time between
DFU_DNLOAD and DFU_GETSTATUS is enough for the device to process the
write. That is, the bwPollTimeout does not effect DFU download if the
first DFU_GETSTATUS after DFU_DNLOAD reports dfuDNLOAD-IDLE state.
Otherwise the host must wait bwPollTimeout ms before issuing next
DFU_GETSTATUS, which slows the communicaiton to not more than 1 download
block (CONFIG_USB_REQUEST_BUFFER_SIZE) every bwPollTimeout ms.

The bwPollTimeout ideally should report an estimate how much longer the
download operation will take. Zephyr does not have such estimate and
therefore defaults to using fixed value. Reduce default bwPollTimeout
from 256 to 8 ms to allow significantly faster DFU downloads on devices
where the time between DFU_DNLOAD and DFU_GETSTATUS is too short to
process download block at the expense of unnecessary bus traffic if
processing download block takes longer than 8 ms.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>