brcmfmac: support repeated brcmf_fw_alloc_request() calls

During a normal brcmfmac lifetime brcmf_fw_alloc_request() is called
once only during the probe. It's safe to assume provided array is clear.

Further brcmfmac improvements may require calling it multiple times
though. This patch allows it by fixing invalid firmware paths like:
brcm/brcmfmac4366c-pcie.binbrcm/brcmfmac4366c-pcie.bin

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>