firmware: fw_base: Remove CSR_MTVEC update check

Remove unnecessary CSR_MTVEC read to reduce the openSBI CSR requirement.

Mux are costly in FPGA. Allowing CSR_MTVEC to be write only is usefull
for the FMax/Area of FPGA softcore.

https://github.com/SpinalHDL/opensbi.git

 branch mtvec

Signed-off-by: Charles Papon <charles.papon.90@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>