PM / OPP: opp_find_freq_exact() documentation fix

opp_find_freq_exact() documentation has is_available instead
of available. This also fixes warning with the kernel-doc:
scripts/kernel-doc drivers/base/power/opp.c >/dev/null
Warning(drivers/base/power/opp.c:246): No description found for parameter 'available'
Warning(drivers/base/power/opp.c:246): Excess function parameter 'is_available' description in 'opp_find_freq_exact'

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>