drivers/rtc/rtc-mxc.c: fix irq enabled interrupts warning

Fixes

  WARNING: at irq/handle.c:146 handle_irq_event_percpu+0x19c/0x1b8()
  irq 25 handler mxc_rtc_interrupt+0x0/0xac enabled interrupts
  Modules linked in:
   (unwind_backtrace+0x0/0xf0) from (warn_slowpath_common+0x4c/0x64)
   (warn_slowpath_common+0x4c/0x64) from (warn_slowpath_fmt+0x30/0x40)
   (warn_slowpath_fmt+0x30/0x40) from (handle_irq_event_percpu+0x19c/0x1b8)
   (handle_irq_event_percpu+0x19c/0x1b8) from (handle_irq_event+0x28/0x38)
   (handle_irq_event+0x28/0x38) from (handle_level_irq+0x80/0xc4)
   (handle_level_irq+0x80/0xc4) from (generic_handle_irq+0x24/0x38)
   (generic_handle_irq+0x24/0x38) from (handle_IRQ+0x30/0x84)
   (handle_IRQ+0x30/0x84) from (avic_handle_irq+0x2c/0x4c)
   (avic_handle_irq+0x2c/0x4c) from (__irq_svc+0x40/0x60)
  Exception stack(0xc050bf60 to 0xc050bfa8)
  bf60: 00000001 00000000 003c4208 c0018e20 c050a000 c050a000 c054a4c8 c050a000
  bf80: c05157a8 4117b363 80503bb4 00000000 01000000 c050bfa8 c0018e2c c000e808
  bfa0: 60000013 ffffffff
   (__irq_svc+0x40/0x60) from (default_idle+0x1c/0x30)
   (default_idle+0x1c/0x30) from (cpu_idle+0x68/0xa8)
   (cpu_idle+0x68/0xa8) from (start_kernel+0x22c/0x26c)

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Sascha Hauer <kernel@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>