staging: rtl8712: usleep_range is preferred over udelay

Fix checkpatch.pl issue: "CHECK: usleep_range is preferred over
udelay;
see Documentation/timers/timers-howto.txt".
Replace `udelay()` with a call to `usleep_range()` with a reasonable
upper limit determined by the other sleeping functions present.

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>