brcmfmac: set F2 watermark to 256 for 4373

We got SDIO_CRC_ERROR with 4373 on SDR104 when doing bi-directional
throughput test. Enable watermark to 256 to guarantee the operation
stability.

Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Wright Feng <wright.feng@cypress.com>
Signed-off-by: Chi-Hsien Lin <chi-hsien.lin@cypress.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>