mmc: Fix potential timer value truncation

On 64bit systems the timer value might be truncated to a 32bit value
causing malfunctions. For example on ARM the timer might start from 0
again only after a cold reset. The 32bit overflow occurs after a bit
more than 49 days (1000 Hz counter) so booting after that time may lead
to a surprise because the board might become stuck requiring a cold
reset.

Signed-off-by: Ronald Wahl <ronald.wahl@legrand.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>