clk: max77686: Make max77686_clk_register() return struct clk *

clk: max77686: Make max77686_clk_register() return struct clk *

As a preparation for further patches, this patch modifies the clock
registration helper function to return a pointer to the newly registered
clock. No functional change is done to the driver.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>