[PATCH] dell_rbu: fix error check

platform_device_register_simple() returns error code as pointer when it
fails.  The return value should be checked by IS_ERR().

Cc: Abhay Salunke <abhay_salunke@dell.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Matt Domsch <Matt_Domsch@dell.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>