clk: ti: dpll: change error return from ~0 to -EINVAL

When the requested rate cannot be achieved, omap2_dpll_round_rate()
will return ~0. Let's change this to -EINVAL to make the code a little
cleaner to read. This is no functional change.

Tested-by: Anddreas Kemnade <andreas@kemnade.info> # OMAP3 GTA04, OMAP4 Panda
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Brian Masney <bmasney@redhat.com>