mmc: core: Remove unused variable

Removed a local variable in mmc_start_req() which was used to store the
error encountered while starting a new data request. If an error is
encountered while starting a new request, the error
information is stored in the mrq. The error information stored in the
mrq is during error handling. So removing the unused variable.

Change-Id: Ifeedc91433230d5156d286bd332a85fb221188ac
Signed-off-by: Vijay Viswanath <vviswana@codeaurora.org>