[flash_ctrl,lint] Fix width mismatch warnings
[flash_ctrl,lint] Fix width mismatch warnings

The idx variable has width WordSelW, which is at least
$clog2(WidthMultiple). Explicitly slice out the relevant bits from
WidthMultiple - 1 to avoid a Verilator width mismatch warning.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan