mtd: Standardising prints in onenand_base.c

This patch resolves all the prints present in onenand_base.c
Primarily, it replaces the hard-coded function names in the prints,
and makes use of __func__.

Signed-off-by: Amul Kumar Saha <amul.saha@samsung.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>