bnxt: apply computed clamp value for coalece parameter

After executing "ethtool -C eth0 rx-usecs-irq 0", the box becomes
unresponsive, likely due to interrupt livelock.  It appears that
a minimum clamp value for the irq timer is computed, but is never
applied.

Fix by applying the corrected clamp value.

Fixes: 74706afa

 ("bnxt_en: Update interrupt coalescing logic.")
Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>