drivers: pinctrl: silabs: Fix multiple analogue bus allocation on same port

Fix ABUS allocation if both EVEN and ODD busses are used for the
same port. The initialisation loop would incorrectly overwrite
entire GPIO_nBUSALLOC when iterating the pinctrl array, must do
a read/mask/update/write sequence.

Signed-off-by: Jonny Gellhaar <jonny.gellhaar@prevas.se>