MIPS: Remove duplicate EBase configuration

Clean up our configuration of the EBase register by making
configure_exception_vector() write to it unconditionally on systems
implementing MIPSr2 or higher, and removing the duplicate code in
per_cpu_trap_init(). The latter would have duplicated work on systems
with vectored interrupts, and didn't set BEV for safety like the
configure_exception_vector() version of the code does.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Cc: linux-mips@vger.kernel.org