fs: fat: usage basename in file_fat_write_at, fat_mkdir

This patch involves no functional change. It is just about code
readability.

Both in file_fat_write_at() and fat_mkdir() the incoming file or directory
path are split into two parts: the parent directory and the base name.

In file_fat_write_at() the value of the variable basename is assigned to
the filename parameter and afterwards the variable filename is used instead
of basename. It is more readable to use the variable basename and leave
filename unchanged.

In fat_mkdir() the base name variable is called directory. This is
confusing. Call it basename like in file_fat_write_at(). This allows to
rename parameter new_directory to directory in the implementation of
fat_mkdir() to match the function declaration.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>