staging: axis-fifo: drop redundant read/write_flags from axis_fifo

staging: axis-fifo: drop redundant read/write_flags from axis_fifo

The driver stores file flags (read_flags/write_flags) in the axis_fifo
struct, duplicating information already available in struct file.

Switch to using f->f_flags directly in read/write paths and open(),
removing the unnecessary fields.

Signed-off-by: Ovidiu Panait <ovidiu.panait.oss@gmail.com>
Link: https://lore.kernel.org/r/20250919195400.3180039-5-ovidiu.panait.oss@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>