mtd: msm_qpic_nand: Support for partial page read - fix

In case the required read length is smaller the mtd->writesize, the whole
erase block is still read but only part of it (the required length) is
returned to the user.
This patch updated the calculation of retlen (number of bytes read).

Change-Id: I377ea88f1c3fa4fc136f09f2e43db4d7ebcd4a1b
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>