usb: u_serial: Fix spin lockup issue

Commit f6fb2128 (usb: tty: Add more buffer space and make the
buffer use direct) has enable USB TTY port's low_latency flag to
true for high throughput data transfer. tty_flip_buffer_push() API
is using this flag to make decision about flushing flip buffer to
line discipline in same caller context (if port->low_latency is set)
or worker thread context (if port->low_latency is not set). This API
also calls (tty->ops->flush_chars) in same context from
n_tty_receive_buf() API which results into spin lockup where this
flag is set. Hence fix this issue by releasing spinlock before
calling tty_flip_buffer_push() based on this flag value.

CRs-Fixed: 768503
Change-Id: I9584668134a121913bf78a133ddfc769b20e4cab
Signed-off-by: Mayank Rana <mrana@codeaurora.org>