[ncm] Treat a zero divisor as indicating no alignment requirements

A zero divisor will currently lead to a 16-bit integer overflow when
calculating the transmit padding, and a potential division by zero if
assertions are enabled.

Avoid these problems by treating a divisor value of zero as equivalent
to a divisor value of one (i.e. no alignment requirements).

Signed-off-by: Michael Brown <mcb30@ipxe.org>