cmd/read.c: Fix checking blk_dread return value

The function blk_dread will return -ENOSYS on failure or on success the
number of blocks read, which must be the number asked to read (otherwise
it failed somewhere).  Correct this check.

Reported-by: Coverity (CID: 166335)
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>