mmc: core: Drop redundant check in mmc_send_hpi_cmd()

There is no point checking if HPI is supported in mmc_send_hpi_cmd() as
mmc_interrupt_hpi(), which is the only caller, already checks if HPI has
been enabled. Therefore, let's drop the check and the corresponding error
path.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>