target/arm: fix incorrect current EL bug in aarch32 exception emulation

The arm_current_el() should be invoked after mode switching. Otherwise, we
get a wrong current EL value, since current EL is also determined by
current mode.

Fixes: 4a2696c0 ("target/arm: Set PAN bit as required on exception entry")
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200328140232.17278-1-changbin.du@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>