ARM: OMAP: dma.c: Remove unused function

Remove the function get_gdma_dev() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Tony Lindgren <tony@atomide.com>