cmd/read.c: Fix checking blk_dread return value

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 <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Bin Meng <[email protected]>
Signed-off-by: Tom Rini <[email protected]>