hwrng: stm32 - fix read of the last word

The stm32_rng_read() function samples TRNG by 4 bytes until at
least 5 bytes are free in the input buffer. The last four bytes
are never read. For example, 60 bytes are returned in case the
input buffer size is 64 bytes.

Read until at least 4 bytes are free in the input buffer. Fill
the buffer entirely in case the buffer size is divisible by 4.

Cc: Oleg Karfich <oleg.karfich@wago.com>
Signed-off-by: Tomas Marek <tomas.marek@elrest.cz>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>