Booting a mx51babbage board with a non-dt kernel leads to the following crash:

Unable to handle kernel NULL pointer dereference at virtual address 0000001c
pgd = 80004000
[0000001c] *pgd=00000000
Internal error: Oops: 5 [#1] SMP ARM
Modules linked in:
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.11.0-next-20130903 #287
task: 9f860000 ti: 9f862000 task.ti: 9f862000
PC is at of_get_next_available_child+0x5c/0x68
LR is at of_get_next_available_child+0x1c/0x68
pc : [<8043ea58>]    lr : [<8043ea18>]    psr: 60000193
sp : 9f863d58  ip : 00000000  fp : 9f863d74
r10: 9f89a010  r9 : 9f862000  r8 : 807bb26c
r7 : 80615d5c  r6 : 00000000  r5 : 60000113  r4 : 00000000
r3 : 00000000  r2 : 808770a4  r1 : 00000011  r0 : 60000113
Flags: nZCv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
Control: 10c5387d  Table: 90004019  DAC: 00000017
Process swapper/0 (pid: 1, stack limit = 0x9f862240)
Stack: (0x9f863d58 to 0x9f864000)

This is caused by commit 8548a464 (ASoC: imx-audmux: Read default
configuration from devicetree).

In order to fix this, add a check for 'of_id' so that
imx_audmux_parse_dt_defaults() only gets called when a dt kernel is running.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>