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

Use ieee80211_channel_to_freq_khz() to calculate channel to frequency to
omit proprietary code.

Signed-off-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Message-ID: <c3a94a403d04ba810cffff8afa734a94a218a4e9.1686166624.git.philipp.g.hortmann@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>