mmc: core: rework mmc_mmc_erase_timeout function

mmc: core: rework mmc_mmc_erase_timeout function

BZ: 29554

The calculation of erase/trim timeout is not excatly the same with
spec. Needs a rework. This patch can fix this.

Change-Id: Ieca799d2f1c181304f5dc57e896114868fe6ed99
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Reviewed-on: http://android.intel.com:8080/41560
Reviewed-by: Tang, Richard <richard.tang@intel.com>
Tested-by: Sun, Jianhua <jianhua.sun@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>