OMAP4: clock: fix compile warning

Fix the following compile warning:

arch/arm/mach-omap2/clock44xx_data.c: In function 'omap4xxx_clk_init':
arch/arm/mach-omap2/clock44xx_data.c:3371:6: warning: 'cpu_clkflg' may be used uninitialized in this function

The approach taken here is intended to work if omap4xxx_clk_init() is
converted into an initcall.

Thanks to Bjarne Steinsbo <bsteinsbo@gmail.com> for proposing another
approach.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Bjarne Steinsbo <bsteinsbo@gmail.com>