[csrng] Fix width of hex constant
[csrng] Fix width of hex constant

The LHS of the comparison is 19 bits wide. This actually has a
parameter already (GenBitsCntrWidth), so use that!

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

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