bridge: cyasgadget, remove file system / vfs calls

This patch remove file system specific (fat_get_block) and vfs calls
from the cyasgadget driver. The current implementation expects user
space to write the file (open, seek to end, 1 byte write), followed by a
call to clear the dirty pages from the the page cache.

Signed-off-by: David Cross <david.cross@cypress.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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