staging: r8188eu: refactor rtw_ch2freq()

staging: r8188eu: refactor rtw_ch2freq()

Convert the array ch_freq_map to a simple integer array and use
the indices as channel numbers. This simplifies the code and avoids
looping through the array to get the frequency. To avoid out of
bounds array access return a default value for invalid channel values,
like the original code did.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20220222082847.6687-3-straube.linux@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>