ath11k: remove redundant assignment to variables vht_mcs and he_mcs

The variables vht_mcs and he_mcs are being initialized in the
start of for-loops however they are re-assigned new values in
the loop and not used outside the loop. The initializations
are redundant and can be removed.

Cleans up clang scan warnings:

warning: Although the value stored to 'vht_mcs' is used in the
enclosing expression, the value is never actually read from
'vht_mcs' [deadcode.DeadStores]

warning: Although the value stored to 'he_mcs' is used in the
enclosing expression, the value is never actually read from
'he_mcs' [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220507184155.26939-1-colin.i.king@gmail.com