mtd: Replace if and BUG with BUG_ON

Replace if condition and BUG() with a BUG_ON having the conditional
expression of the if statement as argument.

The Coccinelle semantic patch used to make this change is as follows:
@@ expression E,f; @@

(
  if (<+... f(...) ...+>) { BUG(); }
|
- if (E) { BUG(); }
+ BUG_ON(E);
)

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>