pwm: mediatek: Fix up PWM4 and PWM5 malfunction on MT7623

Since the offset for both registers, PWMDWIDTH and PWMTHRES, used to
control PWM4 or PWM5 are distinct from the other PWMs, whose wrong
programming on PWM hardware causes waveform cannot be output as expected.
Thus, the patch adds the extra condition for fixing up the weird case to
let PWM4 or PWM5 able to work on MT7623.

v1 -> v2: use pwm45_fixup naming instead of pwm45_quirk
v2 -> v3: add more tags for Reviewed-by, Fixes, and Cc stable

Cc: stable@vger.kernel.org
Fixes: caf065f8 ("pwm: Add MediaTek PWM support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Zhi Mao <zhi.mao@mediatek.com>
Cc: John Crispin <john@phrozen.org>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>