board: siemens: iot2050: Fix logical bug in PG1/PG2 detection

This caused the wrong fdtfile to be set and was failing to apply M.2
settings.

Fixes: badaa1f6 ("boards: siemens: iot2050: Unify PG1 and PG2/M.2 configurations again")
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>