tty: serial: Use GFP_ATOMIC instead of GFP_KERNEL in serial8250_em485_init()

serial8250_em485_init() is supposed to be protected with
p->port.lock spinlock.
This may lead to issues when kmalloc sleeps, so it is better to use
GFP_ATOMIC in this spinlocked context.

Fixes: e490c914

 ("tty: Add software emulated RS485 support for 8250")
Reported-by: Ильяс Гасанов <torso.nafi@gmail.com>
Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>