Protect STM32 CRC device from concurrent accesses.

As we create a spinlocked section that increase with buffer size,
we provide a module parameter to release the pressure by splitting
critical section in chunks.

Size of each chunk is defined in burst_size module parameter.
By default burst_size=0, i.e. don't split incoming buffer.
Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>