OMAP1: McBSP: fix build break for non-multi-OMAP1 configs

Commit 3cf32bba

 ("OMAP: McBSP: Convert
McBSP to platform device model") breaks compilation with non-multi-OMAP1
configs:

  CC      arch/arm/mach-omap1/mcbsp.o
arch/arm/mach-omap1/mcbsp.c: In function 'omap1_mcbsp_init':
arch/arm/mach-omap1/mcbsp.c:384: warning: dereferencing 'void *' pointer
arch/arm/mach-omap1/mcbsp.c:387: error: invalid use of void expression
arch/arm/mach-omap1/mcbsp.c:390: warning: dereferencing 'void *' pointer
arch/arm/mach-omap1/mcbsp.c:393: error: invalid use of void expression

Fix by avoiding NULL dereferences.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Acked-by: Jarkko Nikula <jhnikula@gmail.com>
[tony@atomide.com: updated description not to remove unnecessary branch name]
Signed-off-by: Tony Lindgren <tony@atomide.com>