mmc: core: handle flush requests timeout

According to the eMMC 4.5 spec "Flushing a large amount of cached data may
take very unpredictably long time". Therefore the timeout for FLUSH should
be increased to prevent timeouts.

In case the timeout occurs HPI issued.

CRs-Fixed: 500874
Change-Id: Ib00d087d3fe2fa72f5eac096976d3f24b5e4966a
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>