mmc: sdhci-s3c: Do not allow frequencies higher than requested

This patch modifies sdhci_s3c_consider_clock() to fail if bus clock
being considered can not provide frequency lower or equal requested,
instead of returning the lowest supported.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by; Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <chris@printf.net>