tty: ar933x_uart: remove superfluous assignment of ar933x_uart_driver.nr

The 'nr' field of the 'ar933x_uart_driver' structure
is already initialized with the same value in the static
declaration. Remove the superfluous assignment in the
module init routine.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>