sched: Fix __schedule_bug() output when called from an interrupt

If schedule is called from an interrupt handler __schedule_bug()
will call show_regs() with the registers saved during the
interrupt handling done in do_IRQ(). This means we'll see the
registers and the backtrace for the process that was interrupted
and not the full backtrace explaining who called schedule().

This is due to 838225b4 (sched: use show_regs() to improve
__schedule_bug() output, 2007-10-24) which improperly assumed
that get_irq_regs() would return the registers for the current
stack because it is being called from within an interrupt
handler. Simply remove the show_reg() code so that we dump a
backtrace for the interrupt handler that called schedule().

Change-Id: I2922e793bed4c2bf3f1e5d1be851c8d61a384fee
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>