qlge: Move firmware event handler.

This is not a logical change but rather a move of the inbound firmware event
handler into it's own function as it will later be called by the outbound
path.
The addition of the mutex is to create exclusive access to the mailbox
commands between inbound and outbound handling.

Signed-off-by: Ron Mercer <ron.mercer@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>