staging: rtl8723bs: remove get_monotonic_boottime()

get_monotonic_boottime() is deprecated because it uses the
old 'timespec' structure. This replaces one of the last callers
with a call to ktime_get_boottime, which also simplifies it
a bit by avoiding a double conversion.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Quytelda Kahja <quytelda@tamalin.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>