phy: phy-brcm-usb: select SOC_BRCMSTB on brcmstb only

phy: phy-brcm-usb: select SOC_BRCMSTB on brcmstb only

phy-brcm-usb has some conditional init code required on selected brcmstb
devices. Execution of that code depends on family / product detected by
brcmstb soc code.

For ARCH_BCM4908 brcmstb soc code always return 0 values as ids. Don't
bother selecting & compiling that redundant driver.

Depends-on: 149ae80b1d50 ("soc: bcm: brcmstb: add stubs for getting platform IDs")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210305152406.2588-1-zajec5@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>