Fix compilation of dsp_mem.c. omapfb_ready is only available if 
CONFIG_FB_OMAP_LCDC_EXTERNAL is set, else compiler stops with error. 
status is only used in case CONFIG_FB_OMAP_LCDC_EXTERNAL is set. 
Compiler warns about this.

Signed-off-by: Dirk Behme <dirk.behme@gmail.com>