drivers/rtc/rtc-tps65910.c: fix invalid pointer access on _remove()

The tps65910_rtc data is registered as the platform driver data in
_probe(= ).  Therefore the tps65910_rtc should be used on unregistering
the rtc device.  And device pointer should be retrieved from the
platform_device structure.

This patch fixes the below oops:

 Unable to handle kernel NULL pointer dereference at virtual address 00000008
 Modules linked in: rtc_tps65910(-)
 CPU: 0    Not tainted  (3.7.0-rc7-next-20121128-g6b1f974-dirty #7)
 PC is at tps65910_rtc_alarm_irq_enable+0x20/0x2c [rtc_tps65910]
     (tps65910_rtc_alarm_irq_enable+0x20/0x2c [rtc_tps65910])
     (tps65910_rtc_remove+0x18/0x28 [rtc_tps65910])
     (platform_drv_remove+0x18/0x1c)
     (__device_release_driver+0x70/0xcc)
     (driver_detach+0xb4/0xb8)
     (bus_remove_driver+0x7c/0xc0)
     (sys_delete_module+0x148/0x21c)

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>