watchdog: ar7_wdt.c: use devm_request_and_ioremap

Combine request_region and ioremap into devm_request_and_ioremap.  This has
the effect of fixing a missing iounmap on the failure of clk_get.

This also introduces a call to clk_put and clears the vbus_clk variable in
the case of failure or device removal.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>