pinctrl: renesas: checker: Fix off-by-one bug in drive register check

The GENMASK(h, l) macro creates a contiguous bitmask starting at bit
position @l and ending at position @h, inclusive.

This did not trigger any error checks, as the individual register fields
cover at most 3 of the 4 available bits.

Fixes: 08df16e0 ("pinctrl: sh-pfc: checker: Add drive strength register checks")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/8f82d6147fbe3367d4c83962480e97f58d9c96a2.1633615652.git.geert+renesas@glider.be