pdc202xx_new: PLL detection fix

Fix a bitmask typo in the pdc202xx_new PLL frequency detection code
which causes it to truncate an intermediate difference to 26 bits
instead of the correct 30 bits (the PLL's bitwidth).

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>