RISC-V: Remove redundant err variable

Return value from user_regset_copyin() directly instead of taking this
in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Signed-off-by: CGEL ZTE <cgel.zte@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>