net/fm10k/base: update comment regarding reserved bits check

net/fm10k/base: update comment regarding reserved bits check

The original comment may be read incorrectly as referring to checking
the *entire* length is zero. However, it merely checks only the reserved
bits of both length and reserved in a small amount of code. Update the
comment to indicate this is a clever trick and clearly spell out that it
only checks the reserve bits.

Signed-off-by: Qi Zhang <qi.z.zhang@intel.com>