arch: riscv: fix exception_depth with RISCV_ALWAYS_SWITCH_THROUGH_ECALL

When 'arch_switch()' switches though Ecall, 'exception_depth' is
incorrectly added to the next thread because the current thread is updated
before arch_switch().
Add 'exception_depth' back to the previous thread when Ecall is called from
'arch_switch()'.

Signed-off-by: Jimmy Zheng <jimmyzhe@andestech.com>