fix(io): compare function pointers with NULL

fix(io): compare function pointers with NULL

The ops->read and ops->write existence was checked with 0, change it
to NULL.
This corrects sparse issues:
drivers/io/io_block.c:272:9: warning:
 Using plain integer as NULL pointer
drivers/io/io_block.c:384:9: warning:
 Using plain integer as NULL pointer
drivers/io/io_block.c:384:9: warning:
 Using plain integer as NULL pointer

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I039050a645107523d8263ddf820e539c260d956c