fix get_monotonic_boottime is deprecated in kernel 4.20.

[m-gardet]

https://www.kernel.org/doc/html/latest/core-api/timekeeping.html#deprecated-time-interfaces

ps : fix for issue https://github.com/whitebatman2/rtl8821CU/issues/19

[RobMurray98]

This worked for me installing on kernel 5.0.0.23, would like to see it merged in as it caused me a big headache!