staging: rtl8192u: propagate errors in eprom_read

Propagate error from eprom_r and change the return type of eprom_read
from u32 to int.

Signed-off-by: Salah Triki <salah.triki@acm.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>