mmc: mmc_spi: fix timeout calculation

Previously the cycle timeout was converted to a microsecond value but
then incorrectly treated as a nanosecond timeout. This patch changes
the code to convert both the nanosecond timeout and the cycle timeout
to a microsecond value and use that directly.

Signed-off-by: Tobias Schramm <t.schramm@manjaro.org>
Link: https://lore.kernel.org/r/20200814185011.3252020-1-t.schramm@manjaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>