mmc: core: fix power class selection

mmc_select_powerclass() function returns error if eMMC
VDD level supported by host is between 2.7v to 3.2v.

According to eMMC specification, valid voltage for high
voltage cards is 2.7v to 3.6v. This patch ensures that
2.7v to 3.6v VDD range is treated as valid range.

Also, failure to set the power class shouldn't be treated
as fatal error because even if setting the power class
fails, card can still work in default power class.
If mmc_select_powerclass() returns error, just print
the warning message and go ahead with rest of the card
initialization.

Change-Id: I3371458d852a05518b966041775fff59c04d255c
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>