[ARM] omap: fix _omap2_clksel_get_src_field()

_omap2_clksel_get_src_field() was returning the first entry which was
either the default _or_ applicable to the SoC.  This is wrong - we
should be returning the first default which is applicable to the SoC.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>