staging: rtl8192e: Use standard api to calculate frequency to channel

Use ieee80211_freq_khz_to_channel() to calculate frequency to channel to
omit proprietary code.

Signed-off-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Message-ID: <fc332e81f6677965e9e51f1fcef9815997effcb5.1686166624.git.philipp.g.hortmann@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>