firewire: core: code cleanup after vm_map_pages_zero introduction

Commit 22660db8

 turned fw_iso_buffer_map_vma into a one-liner.
There is no need to keep this in the core-iso.c collection of buffer
management functions; put it inline into the sole user, the character
device file driver.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>