ACPI: SBS: remove unused const variable 'SMBUS_PEC'

drivers/acpi/sbshc.h:18:17: warning:
 SMBUS_PEC defined but not used [-Wunused-const-variable=]

SMBUS_PEC is never used since introduction in
commit 91087dfa ("ACPI: SBS: Split host
controller (ACPI0001) from SBS driver (ACPI0002)"),
so just remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>