brcmfmac: introduce brcmf_fw_alloc_request() function

The function brcmf_fw_alloc_request() takes a list of required files
and allocated the struct brcmf_fw_request instance accordingly. The
request can be modified by the caller before being passed to the
brcmf_fw_request_firmwares() function.

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>