pinctrl: sh-pfc: checker: Improve pin checks

Improve the checks for pin descriptors:
  1. Introduce local variables for the current pin, to make the checks
     easier to read,
  2. Pins must have a name,
  3. Fix double printing of identical pin names.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200110131927.1029-7-geert+renesas@glider.be