Including nvme_spec.h in a C++ source file and compiling with -Wall
results in an invalid conversion warning when implicitly casting
from int to enum spdk_nvme_data_transfer. So add an explciit cast
to silence the warning.
Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: I54fefeb03b13ff47baa2bcff830330c2b2963493