hwrng: xilinx-trng - propagate timeout before any data is read

In the Linux kernel, the following vulnerability has been resolved:

hwrng: xilinx-trng - propagate timeout before any data is read

xtrng_readblock32() polls for 16-byte chunks but returns the number of
bytes read even when the first poll times out. Its caller then treats a
zero return as a short successful read, and partial reads for full
32-byte blocks can make the tail copy use a fixed block offset rather
than the amount already produced.

Return the poll error when no data has been read, preserve partial
positive returns after some data is available, stop the generator on all
collection exits, and append tail bytes at the current output count.