Staging: rtl8188eu: rtw_pwrctrl: fixed checkpatch warning.

WARNING: msleep < 20ms can sleep for up to 20ms; see Documentation/timers/timers-howto.txt

Signed-off-by: Surya Seetharaman <suryaseetharaman.9@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>