rtlwifi: set proper udelay within rf_serial_read

Since read RF register is an indirect access that hardware needs time to
accomplish read action, but there's no ready bit, so delay is required to
guarantee the read value is correct. After investigating internal documents,
these delays are reduced as proper values.

Reported-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>