drivers: serial: uart_sam0: Correct check for interrupts enabled
drivers: serial: uart_sam0: Correct check for interrupts enabled

Fixes the issue where uart_sam0_irq_tx_ready would return true if
the INTFLAG was set even though the INTSET bit for the given
interrupt was not enabled yet through uart_sam0_irq_tx_enable.

Signed-off-by: Ron Smith <7b5fcf60d6cb3c8c31ce793bddd1fbe10bdc1004@gmail.com>

zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr