Building the xr_usb_serial module fails for Kenel 5.10 with CONFIG_PM
enabled:

xr_usb_serial_common.c:1574:15: error: 'ASYNCB_INITIALIZED' undeclared
(first use in this function); did you mean 'RCU_INITIALIZER'?

Use tty_port_initialized in order to determine the status of the TTY
port. This is compatible with Kernel 5.10 and at least Kernel 5.4.

Signed-off-by: David Bauer <mail@david-bauer.net>

Cleaned up Makefile to modern standards.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

The SUBDIRS variable has been removed in kernel 5.4, and was deprecated
since the beginnig of kernel git history in favour of M or KBUILD_EXTMOD.

Signed-off-by: David Bauer <mail@david-bauer.net>

Signed-off-by: Daniel Golle <daniel@makrotopia.org>