Update am_hal_uart.c

[RichardSWheatley]

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The call to am_hal_pwrctrl_periph_disable will leave some values of the UARTn->CR register powered up and the current draw to be 200-300nA higher than expected. The below adjustment is acceptable since the pState->sRegState.regCR will retain the state and return the state if requested..

UARTn(ui32Module)->CR = 0;