Check proper value in assert

The different kernels does a lot of parameter checking, and one of the
checks for the FP16 kernels is actually incorrect. This change addresses
one error of this kind in the fp16 lhs packing kernel

The same issue is also updated in the fp16 documentation

Signed-off-by: Emil Ohlsson <emil.ohlsson@arm.com>

Reviewed-by: Felix Johnny Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>
Approved-by: Felix Johnny Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>