serial: pch_uart: Fix signed-ness and casting of uartclk related fields

Storing one struct per known board would be overkill. Pre-cast the
driver_data pointer to an unsigned long to avoid the pointer to int
compiler warning:

drivers/tty/serial/pch_uart.c:431:10: warning: cast from pointer to
integer of different size [-Wpointer-to-int-cast]

Unify the signed-ness of the baud and uartclk types throughout the
driver.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>