Mk/Uses/npm.mk: Unbreak make index

In the previous commit, NPM_VER_MAJOR and NPM_VER_MINOR are left
undefined when NPM_VER is manually specified. This change corrects the
error.

Reported by: Xavier Humbert <xavier@groumpf.org> (via ports),

		antoine (via ports-committers)

Fixes: dad28f35db5a (Mk/Uses/npm.mk: Correct NPM version comparison)