mmc: bcm2835_sdhci: Restore original delay behavior

Patch 33fe2fb8 titled
"mmc: bcm283x: Remove get_timer_us() from mmc driver"
incorrectly replaced ad-hoc get_timer_us() function
with a plain get_timer(). The get_timer() operates in
mSec units instead of uSec though, which caused very
slow operation of the driver.

Restore the original behavior of the driver, but avoid
get_timer_us() and use timer_get_us() instead. The later
is part of the standard API.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jakub Kiciński <moorray3@wp.pl>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Tested-by: Jakub Kicinski <kubakici@wp.pl>