ath11k: Remove htt stats fixed size array usage

To support the HTT Stats DebugFS interface a single large buffer that
contains the stats must be provided to the DebugFS infrastructure.
In the current code, for each class of stats, the stats are first
formatted in a local on-stack buffer, and then the local buffer is
copied to the large DebugFS buffer.

This logic has a problem when, for a given class, the formatted
stats exceed the size of the on-stack buffer. When this occurs the
stats for this class is truncated. In addition, this logic is
inefficient since it introduces an unnecessary memory copy.

To address these issues, update the logic to no longer use a local
on-stack buffer, and instead write the formatted data directly into
the large DebugFS buffer.

Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.5.0.1-01105-QCAHKSWPL_SILICONZ-1
Signed-off-by: Seevalamuthu Mariappan <seevalam@codeaurora.org>
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210913223148.208026-4-jouni@codeaurora.org