firmware-samsung-i9100: place firmware in /lib/firmware/brcm

The kernel looks for the firmware files in /lib/firmware/brcm, not in /lib/firmware/postmarketos/brcm. Fix the path so that the kernel can automatically find the right firmware.

(The other fix would be to make the kernel look for the firmware there, but 1. that requires another patch, and 2. firmware-samsung-midas and firmware-samsung-lt01 both place WiFi firmware directly in /lib/firmware/brcm and it works fine for them.)