mmc: sdhci-s3c: Use correct condition to check for clock presence

IS_ERR() must be used to make sure that not a valid clock was returned
by clk_get() and company.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
mmc: sdhci-s3c: Simplify min/max clock calculation

This patch reimplements functions calculating minimum and maximum clock
rates to leverage clock rate cache introduced by previous patches.

In addition, the calculation is simplified to just comparing input
clock rates (max case) or input clock rates divided by maximum divisor
(min case), which is basically what the original code did, but with much
more unnecessary work.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
mmc: sdhci-s3c: Fix handling of bus clock switching

Currently the driver assumes at probe that controller is configured for
last valid enumerated bus clock. This assumption is completely wrong, as
there is no way to ensure such configuration until the hardware gets
first configured (by calling sdhci_s3c_set_clock()).

This patch modifies the driver to set current clock at probe to unknown
state (represented by negative value) and make sure that the hardware
gets actually configured to selected clock in sdhci_s3c_set_clock().

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
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>