spmi: pmic-arb: u8 <= 0xff is always true

Silences this static checker warning:

 drivers/spmi/spmi-pmic-arb.c:363
 pmic_arb_write_cmd() warn: always true condition
 '(opc <= 255) => (0-255 <= 255)'

Cc: Andy Gross <agross@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>