eeprom: idt_89hpesx: remove redundant variable csrval_len

Variable csrval_len is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
warning: variable 'csrval_len' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>