drivers/tty/serial: delete unused MODULE_DEVICE_TABLE from atmel_serial.c

In commit c39dfebc ("drivers/tty/serial:
make serial/atmel_serial.c explicitly non-modular") we removed the code
relating to modular support since it currently only supports built in.

However, when redoing my build coverage for mips allmodconfig, which
sets CONFIG_OF, I noticed a remaining line that needs to be removed,
else we will get a build failure for an undefined module macro.

Unfortunately this didn't appear for any of the other arch I tested
more frequently, such as ARM.

Since MODULE_DEVICE_TABLE is a no-op for non-modular code, we can just
remove the offending line.

Fixes: c39dfebc ("drivers/tty/serial: make serial/atmel_serial.c explicitly non-modular")
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Jiri Slaby <jslaby@suse.com>
Reported-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>