fix(rockusb/nusb): Bulk reads should request a length that is a multiple of EP max packet size

[dingari]

This change was present in #47 but seems to have been orphaned in the switch to nusb 0.2.

As per the nusb docs:

For an IN transfer, the buffer’s requested_len is the number of bytes requested. It must be a nonzero multiple of the endpoint’s maximum packet size or the transfer will fail with TransferError::InvalidArgument.

Confirmed to be an issue at least on macOS.