tty: serial: constify dev_pm_ops structures

Declare dev_pm_ops structures as const as they are only stored in the pm
field of a device_driver structure. This field is of type const, so
dev_pm_ops structures having similar properties can be declared const
too.

Size details after cross compiling the .o file for blackfin architecture.

File size before:
  text	   data	    bss	    dec	    hex	filename
   3572	    320	     16	   3908	    f44 tty/serial/bfin_sport_uart.o

File size after:
   text	   data	    bss	    dec	    hex	filename
   3664	    228	     16	   3908	    f44 tty/serial/bfin_sport_uart.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>