ARM: mmc: bcm283x: Remove get_timer_us() from mmc driver

The get_timer_us() function is something which is no longer
existing in case we use generic timer framework, so replace
it with get_timer().

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Tyler Baker <tyler.baker@linaro.org>