stm32f4: fix incorrect baud rate calculation for USART2

USART1 and USART6 are on APB2 whose frequency is 84MHz,
while USART2, USART3, UART4, and UART5 are on APB1 whose
frequency is 42
MHz (max).

Commented-by: Shih-Po Hung <arcbbb@gmail.com>
Signed-off-by: Jim Huang <jserv@0xlab.org>