ata: ahci_brcmstb: Fix misuse of IS_ENABLED

While IS_ENABLED() is perfectly fine for CONFIG_* symbols, it is not
for other symbols such as __BIG_ENDIAN that is provided directly by
the compiler.

Switch to use CONFIG_CPU_BIG_ENDIAN instead of __BIG_ENDIAN.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org