[BUG: #244] userspace/wrsw_hal: read only real-time data for SFP diagnostics

This commit reduces the size of periodic read for each SFP from 256B to 16B.
Since the I2C transfer are bit-banged, this commit reduces CPU
utilization if monitoring of SFPs is used (CONFIG_READ_SFP_DIAG_ENABLE).

Signed-off-by: Adam Wujek <dev_public@wujek.eu>