ath10k: pci: Fix comment on ath10k_pci_dump_memory_sram

The description of ath10k_pci_dump_memory_sram() is inaccurate, an error
can never be returned, it is always the length. Update the comment to
reflect.

Fixes: 219cc084 ("ath10k: add memory dump support QCA9984")
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>