watchdog: sp805_wdt.c: use clk_prepare_enable and clk_disable_unprepare
Clk_prepare_enable and clk_disable_unprepare combine clk_prepare and
clk_enable, and clk_disable and clk_unprepare. They make the code more
concise, and ensure that clk_unprepare is called when clk_enable fails.
A simplified version of the semantic patch that introduces calls to these
functions is as follows: (http://coccinelle.lip6.fr/)