mmc: check for > 1 clk_requests when disabling gating

Since we make sure the clock is enabled in the mmc_host_clk_exit()
function we should expect a reference counter of 1, not 0.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Tested-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Chris Ball <cjb@laptop.org>