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: NИльяс Гасанов <torso.nafi@gmail.com>
Signed-off-by: NMatwey V. Kornilov <matwey@sai.msu.ru>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>