IXP: Fix NAND build warning on PDNB3 and SCPU

IXP: Fix NAND build warning on PDNB3 and SCPU

nand.c: In function ‘pdnb3_nand_read_buf’:
nand.c:107:4: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
nand.c: In function ‘pdnb3_nand_dev_ready’:
nand.c:124:18: warning: variable ‘val’ set but not used [-Wunused-but-set-variable]

Remove the 4-byte-at-time read mode altogether, the most is bogus and will
likely cause unaligned accesses.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Bryan Hundven <bryanhundven@gmail.com>
Cc: Michael Schwingen <rincewind@discworld.dascon.de>