pwm: img: Turn final 'else if' into 'else' in img_pwm_config

When building with -Wsometimes-uninitialized, Clang warns:

drivers/pwm/pwm-img.c:126:13: error: variable 'timebase' is used
uninitialized whenever 'if' condition is false
[-Werror,-Wsometimes-uninitialized]

The final else if functions as an else; make that explicit so that Clang
understands that timebase cannot be used uninitialized.

Link: https://github.com/ClangBuiltLinux/linux/issues/400
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>