[ARM] omap: fix a pile of issues

This patch fixes a number of sillies, from missing 'const' to using
'return' in void functions, to functions with no arguments not even
'void' and a cast which isn't required.

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>