mmc: sdhci-of-at91: fix memleak on clk_get failure

sdhci_alloc_host() does its work not using managed infrastructure, so
needs explicit free on error path. Add it where needed.

Cc: <stable@vger.kernel.org>
Fixes: bb5f8ea4 ("mmc: sdhci-of-at91: introduce driver for the Atmel SDMMC")
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/b2a44d5be2e06ff075f32477e466598bb0f07b36.1577961679.git.mirq-linux@rere.qmqm.pl


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