drivers: entropy: stm32: don't waste generated random data

Even though the STM32 TRNG hardware produces 2- or 4-byte sized words of
random data before triggering an interrupt, the driver currently discards
all but the bottom byte: 50/75% of the produced entropy goes to waste!

Make sure we consume all the random data from each word we read to improve
the entropy generation rate seen by users of the driver.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>