clocksource/drivers/fsl_ftm_timer: Fix error return checking

The error checks on freq for a negative error return always fails because
freq is unsigned and can never be negative. Fix this by making freq a
signed long.

Detected with Coccinelle:
drivers/clocksource/fsl_ftm_timer.c:287:5-9: WARNING: Unsigned expression
compared with zero: freq <= 0
drivers/clocksource/fsl_ftm_timer.c:291:5-9: WARNING: Unsigned expression
compared with zero: freq <= 0

Fixes: 2529c3a3 ("clocksource: Add Freescale FlexTimer Module (FTM) timer support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: kernel-janitors@vger.kernel.org
Link: https://lkml.kernel.org/r/20180226113614.3092-1-colin.king@canonical.com