pwm: loongson: Fix LOONGSON_PWM_FREQ_DEFAULT

Per the 7A1000 and 7A2000 user manual, the clock frequency of their
PWM controllers is 50 MHz, not 50 kHz.

Fixes: 2b62c894 ("pwm: Add Loongson PWM controller support")
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Reviewed-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://lore.kernel.org/r/20250816104904.4779-2-xry111@xry111.site


Cc: stable@vger.kernel.org
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>