mtd: spi-nor: Fix style issues in spi-nor.c

Fix bp_bits_from_sr() that is not KNF.
Move lock_granularity = max(..) to else instead of doing each pass.
Fix minor typo (Mircon -> Micron).

Signed-off-by: William Bourque <wfb@xiphos.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>