clocksource: nps: avoid maybe-uninitialized warning

We get a harmless false-positive warning with the newly added nps
clocksource driver:

drivers/clocksource/timer-nps.c: In function 'nps_setup_clocksource':
drivers/clocksource/timer-nps.c:102:6: error: 'nps_timer1_freq' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Gcc here fails to identify that IS_ERR() is only true if PTR_ERR()
has a nonzero value. Using PTR_ERR_OR_ZERO() to convert the result
first makes this obvious and shuts up the warning.

Fixes: 0ee4d9922df5 ("clocksource: Add clockevent support to NPS400 driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>