fs/fat: avoid noisy message fat_read_file()

fs/fat: avoid noisy message fat_read_file()

UEFI applications call file system functions to determine if a file exists.
The return codes are evaluated to show appropriate messages.
U-Boot's file system layer should not interfere with the output.

Rename file_fat_read_at() to fat_read_file() adjusting the parameter
sequence and names and eliminate the old wrapper function.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>