mmc: sdhci: clean up timeout detection

The current timeout detection logic is not very nice; it calls
get_timer(start) in the while() loop, and then calls it again after
the loop to check if a timeout error happened.

Because of the time difference between the two calls of get_timer(),
the timeout detected after the loop may not be true.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>