imx: timer: don't clear the GPT control register multiple times

imx: timer: don't clear the GPT control register multiple times

There is no need to clear the control register 100 times in a
loop, a single zero write clears the register. I didn't find any
justification why clearing this register in a loop is needed
(no info in i.MX6 errata or GPT timer linux driver, linux driver
uses single write to clear this control register).

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>