serial: pxa: Staticize local symbols

These local symbols are used only in this file.
Fix the following sparse warnings:

drivers/tty/serial/pxa.c:793:17: warning: symbol 'serial_pxa_pops' was not declared. Should it be static?
drivers/tty/serial/pxa.c:971:12: warning: symbol 'serial_pxa_init' was not declared. Should it be static?
drivers/tty/serial/pxa.c:986:13: warning: symbol 'serial_pxa_exit' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>