Input: sentelic - fix error return when fsp_reg_write fails

Input: sentelic - fix error return when fsp_reg_write fails

Currently when the call to fsp_reg_write fails -EIO is not being returned
because the count is being returned instead of the return value in retval.
Fix this by returning the value in retval instead of count.

Addresses-Coverity: ("Unused value")
Fixes: fc69f4a6af49 ("Input: add new driver for Sentelic Finger Sensing Pad")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20200603141218.131663-1-colin.king@canonical.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>