spl: fit: Fix the number of bytes read in raw mode

In raw mode a full sector is to be read even if image covers part of
a sector. Number of sectors are calculated as ROUND_UP(size)/sec_size by FIT
framework. This calculation assumes that image is at the 0th offset of a sector,
which is not true always in FIT case. So, include the image offset while
calculating number of sectors.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>