#12 CPR support for 16C950 UART

[Anonymous]

This patch adds 2 ioctl's for reading/writing the content of the CPR (Clock Prescaler Register) in 16C950 UARTs. TIOCGSERCPR reads the value, and returns it to an unsigned char. If the prescaler flag is disabled in MCR, it returns 8 (i.e. no prescaling). TIOCSSERCPR writes the value from an unsigned char and writes it to CPR. If the value is invalid (<8) it disables the prescaler flag in MCR.

The UART is not reset anymore when the serial line is opened, because this would overwrite the CPR. Also, when the serial line is opened, the value of the prescaler flag in MCR is preserved, rather than zeroed.

TIOCGSERCPR and TIOCSSERCPR must be added in asm/ioctls.h, otherwise the thing will not compile.