brcmfmac: add comment block in brcmf_sdio_buscore_read()

In brcmf_sdio_buscore_read() there is some special handling upon
register access to chipid register of the chipcommon core. Add
comment explaining why it is done here.

Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>