drivers: serial: esp32_usb: fix interrupt-enable race condition

After enabling interrupts, serial_esp32_usb_irq_tx_enable() calls the
interrupt handler to handle any old events that occurred while interrupts
were disabled.  However, this happens in the callers context, so if an
interrupt arrives during the call, it will call the interrupt handler again
resulting in corruption if the interrupt handler is not reentrant.

Fixes #74569

Signed-off-by: Eric Holmberg <eric.holmberg@northriversystems.co.nz>