Fixes gcc '-Wunused-but-set-variable' warning:

drivers/mtd/devices/pmc551.c: In function pmc551_erase:
drivers/mtd/devices/pmc551.c:142:15: warning: variable soff_lo set but not used [-Wunused-but-set-variable]
drivers/mtd/devices/pmc551.c: In function pmc551_read:
drivers/mtd/devices/pmc551.c:232:15: warning: variable soff_lo set but not used [-Wunused-but-set-variable]
drivers/mtd/devices/pmc551.c: In function pmc551_write:
drivers/mtd/devices/pmc551.c:289:15: warning: variable soff_lo set but not used [-Wunused-but-set-variable]

It is not used since commit cdf0a7d1 ("[MTD]
pmc551 whitespace cleanup")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>