bnxt_en: Add bnxt_set_max_func_irqs().

By refactoring existing code into this new function.  The new function
will be used in subsequent patches.

v2: Fixed compile warning when CONFIG_BNXT_SRIOV is not set.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>