mt76: mt7915: improve starec readability of txbf

1. Drop unnecessary MT_EBF/MT_IBF enum.
2. Rename fields 'nc'/'nr' to 'ncol'/'nrow'.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>