serial: mvebu-uart: correctly calculate minimal possible baudrate

For default (x16) scheme which is currently used by mvebu-uart.c driver,
maximal divisor of UART base clock is 1023*16. Therefore there is limit for
minimal supported baudrate. This change calculate it correctly and prevents
setting invalid divisor 0 into hardware registers.

Signed-off-by: Pali Rohár <pali@kernel.org>
Fixes: 68a0db1d ("serial: mvebu-uart: add function to change baudrate")
Link: https://lore.kernel.org/r/20210624224909.6350-4-pali@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>