bwi: Remove write-only variable

bbp_atten is write-only: we don't use it later in
bwi_rf_calc_nrssi_slope_11b. However, we may also be able to eliminate
the read of this value from the device. However, The documationation for
this hardware is thing and I can't test this card so I've left the read
of this register in case it's required.

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