[tlul] Explicitly cast between SRAM width and TL width
[tlul] Explicitly cast between SRAM width and TL width

OTBN uses tlul_adapter_sram with a width of 256 bits: more than the TL
bus width of 32 bits. This triggers a couple of Verilator warnings
caused by width mismatches. This patch makes the "extract bottom bits"
and "zero extend" steps explicit. There should be no change in
behaviour.

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

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