usb: cdc_acm: trigger TX IRQ when CDC ACM class is ready

Before commit 4f2682bd
("usb: cdc_acm: avoid spurious interrupt on configured or resume
events") spurious rx-interrupt provided start to read shell tx ring
buffer by CDC ACM UART. Now shell log backend not transmit data,
until CDC ACM UART received data from host.

This PR added invoking interrupt callback if tx interrupt is enable,
when CDC ACM class go to configured state, and flag tx_ready go to true.

Signed-off-by: Grixa Yrev <GrixaYrev@yandex.ru>