[dv] Make width conversion explicit in dv_base_env_cfg::initialize
[dv] Make width conversion explicit in dv_base_env_cfg::initialize

This is needed if BUS_AW doesn't equal `UVM_REG_ADDR_WIDTH. The
implicit (unsigned) conversion will do the right thing if
csr_base_addr is a fixed value, but will zero-extend '1 to something
like 0x00000000ffffffff if BUS_AW = 32 and `UVM_REG_ADDR_WIDTH == 64,
which breaks the magic address handling in set_base_addr.

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

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