arch-riscv: Fixed the style of stats variable names in TlbStats

arch-riscv: Fixed the style of stats variable names in TlbStats

The variable names should be of camel case style.

Change-Id: I397c9f165a53ecc120ec57f7214c90a65e12407e
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40695
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>