tty: serial: 8250: use 32bit variable for rpm_tx_active

The kbuild test robot wrote me:
|  make.cross ARCH=powerpc
|>> ERROR: ".__xchg_called_with_bad_pointer" [drivers/tty/serial/8250/8250.ko] undefined!

The generic implementation of xchg() and arm and x86 specific one work
for variables of size one bye (char). According to the report powerpc does
only support xchg() for 1 byte variables and looking further it seems to
be the same case for sparc or tile (or for 10 out of 26 architectures
which provide a custom implementation).
For that reason I increase the size of the variable from one to four
bytes.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>