ArmPkg/GenericWatchdogDxe: clean up the code

ArmPkg/GenericWatchdogDxe: clean up the code

Clean up the code, by adding missing STATIC modifiers, drop
redundant casts, and get rid of the 'success handling' anti
pattern in the entry point code.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>